What do I actually need to know about how my computer stores things?

Files are named blobs of data that do nothing until an app reads them. Folders nest inside folders to make a tree. A path is a file's address through that tree. That's the whole model, and it's enough to work with Claude on real files.

You double-click a song file. What's actually playing the music?

A music app is. The file is bytes sitting on a disk and can't do anything on its own — your operating system looks at what kind of file it is, finds an app that reads that kind, and hands it over. This is worth internalising because it explains the rest: a file is never doing something, something is always doing it to the file.

I have two browser windows open. Is that one program or two?

One program file on your disk, two processes in memory. Opening it twice loaded the same instructions twice; nothing was duplicated on disk. That distinction explains why your machine slows down when you 'only have one app open', and why closing a window doesn't always quit the app — on Mac you have to Quit it to stop the running copy.

If I rename `song.mp3` to `song.pdf`, does it become a PDF?

No. Nothing at all happens to the contents — you've changed the label and lied to the computer about what's inside. A PDF reader will fail on it because the bytes are still an mp3. Extensions are hints, not converters; actually converting a file needs an app that reads one format and writes the other.

Where does a file I dragged onto my desktop actually live?

In a perfectly ordinary folder called Desktop inside your home folder. The desktop isn't a special area outside the file system — the icons you see are that folder's contents painted onto your wallpaper. Which means you can open it in Claude exactly like any other folder.

Files, folders and paths — Claude for Humans

There’s a category of confusion that has nothing to do with AI, or with Claude, or with code, and it quietly blocks people anyway. You’re asked to “open a folder”, or told the file is at src/index.js, or you see .env and don’t know whether that’s a filename or a typo. None of it is hard. It’s just never been explained to you, because everyone who could explain it learned it so long ago they’ve forgotten it was ever a thing to learn.

This page is that explanation. It takes about ten minutes and it stops being a source of friction permanently.

A file is a name with bytes attached

Forget the office metaphor — paper, manila folders, filing cabinets. On a computer:

A file is a chunk of data with a name on it.

The chunk might be the pixels of a photo, the words of a contract, the rows of a spreadsheet, or the instructions that make an app run. Your computer doesn’t care which. What it tracks is that the thing has a name, has contents, has a size, and has a location.

And here is the part that quietly fixes half of everything else: a file does nothing on its own. A photo isn’t being looked at. A song isn’t playing. A document isn’t open. A file sits on a disk, inert, until something reads it.

When you double-click a photo, your computer doesn’t wake the file up. It looks at what kind of file it is, finds an app that can read that kind, launches the app, and hands the file over. The app did the work. The file never moved.

That’s also why “Open With…” exists. Open the same .jpg in Photos, in a browser, in Photoshop, or in a text editor and you get four different results from identical bytes — including, in the text editor’s case, a screenful of garbage. You didn’t change the file. You changed the reader.

Folders nest, and that’s the whole shape

A folder is a labelled container. It holds files. It also holds other folders — and that second part is the entire structure of your computer.

Documents/
├── work/
│   ├── 2025/
│   │   └── report.pdf
│   └── contracts/
└── personal/
    └── photos/
        └── beach.jpg

Boxes inside boxes. The technical name for this shape is a tree, and your machine is one enormous one with files hanging off the branches.

Two footnotes worth having. First, “folder” and “directory” are the same thing — “folder” is the friendly word your file manager uses, “directory” is the older technical one you’ll see in documentation and in the terminal. Nothing distinguishes them. Second, a folder isn’t really a place on the physical disk; it’s a label saying “these belong together”. That’s why moving a file between folders on the same disk is instantaneous even when the file is enormous — almost nothing physically moves.

A path is an address

You have hundreds of thousands of files. “The report” doesn’t identify one. An address does.

Documents/work/2025/report.pdf

Read left to right: start in Documents, go into work, then into 2025, and the file is report.pdf. Each / means “now go inside this one”. You already walk paths constantly — every time you click a folder, then a folder, then a file, you’re walking one. A written path is that same journey put into words so it can be handed to someone else.

The starting point differs by operating system, which is the only real difference between them here:

Mac:     /Users/you/Documents/report.pdf
Linux:   /home/you/Documents/report.pdf
Windows: C:\Users\you\Documents\report.pdf

Different roots, same idea. Windows historically uses backslashes and accepts forward slashes now; this site uses forward slashes throughout, and so does Claude.

This is the single most useful thing on the page for working with Claude. A path removes ambiguity. “Look at the server file” makes Claude guess; “look at src/server/index.js” doesn’t. Most requests that fail on the first try fail because they described a file instead of naming one.

Extensions are hints, not rules

The bit after the last dot — .pdf, .jpg, .csv, .js — is the file extension, and it suggests what’s inside. Your operating system uses it to pick a default app.

But it only describes the file; it doesn’t make the file anything. Rename song.mp3 to song.pdf and you have an mp3 wearing a false label. Your PDF reader will fail on it, because the bytes were never a PDF. This is why “just change the extension” is almost never how you convert a file — converting needs an app that reads one format and writes the other.

One default worth changing your mental model for: Finder and File Explorer hide most extensions. You see report and report; the real filenames are report.pdf and report.docx. A remarkable amount of “I can’t find the file” is this. In a terminal, and in Claude’s file listings, extensions are never hidden — what you see is what’s there.

The program on disk vs. the thing that’s running

A program is a file too — one whose contents are instructions rather than pixels or words. Chrome, Word, Claude Code, the terminal itself: each is a program file sitting on a disk.

The distinction that matters is between the file and the running copy of it, which is called a process. One program file, many processes: two browser windows are one program running twice. The instructions were loaded into memory twice; the file was never duplicated.

This explains several things at once — why your machine slows down when you “only have one app open”, why closing a window doesn’t always quit the app (especially on Mac, where you have to Quit it), and why a server has to be running for a site to load. When Claude says it started the dev server, it means a process now exists and will keep existing until something stops it.

Where your things actually live

Almost everything you own sits in one folder: your home folder. /Users/you on Mac, C:\Users\you on Windows, /home/you on Linux. Desktop, Documents, Downloads and the rest are ordinary folders inside it — including Desktop, which is not a magic surface but a folder whose contents are painted onto your wallpaper.

Outside your home folder is the operating system’s own territory — /System, /Library, C:\Windows. That’s the computer’s stuff, not yours, and the OS defends most of it. For virtually everything you’ll do with Claude, you’re working inside your home folder.

One caveat that catches people, and it’s worth knowing before it bites: if you use iCloud Drive, Dropbox, OneDrive or Google Drive, some of your “files” aren’t on this computer. They appear in a folder and look completely normal, but the bytes live on a server and get downloaded on demand — sometimes there’s a local copy, sometimes only a placeholder. So when Claude reads one it may have to wait for a download; a file you edit here will eventually change on every other device; and if the sync service is paused or signed out, the file may not be readable at all. If a folder behaves strangely — slow reads, files that seem to be there but aren’t — check whether it’s a synced folder before you look for a stranger explanation.

Every OS, same ideas

Mac, Windows and Linux differ in wallpaper, in where the close button sits, in which slash they prefer and where your home folder starts. Underneath they hold identical concepts: files, folders, trees, paths, extensions, processes.

The one practical consequence for this site is availability. Claude Code Desktop ships for macOS and Windows, and since June 2026 for Ubuntu 22.04+ and Debian 12+ in beta. On any other Linux you reach exactly the same Claude through the terminal instead — which is what desktop vs terminal is about, and it’s a difference of door, not of building. The hands-on version of that door is the Power Track.

That’s the whole foundation. From here, the words people use around software — frontend, backend, API, server — are covered in software words you’ll hear, and pointing Claude at a real file you’ve inherited is understanding code you didn’t write.

The words

Operating system OS: The program that runs your computer and every other program on it — macOS, Windows, Linux. It owns the screen, the disk, the keyboard and the network, and hands out access to them: when you double-click a file, the OS is what decides which app opens it.
File path path: The address of a file, written as the chain of folders you'd walk through to reach it, separated by slashes: Documents/work/2025/report.pdf. Read it left to right — each step goes one folder deeper, and the last piece is the file itself.
File extension: The short tag after the last dot in a filename — .pdf, .jpg, .csv, .js. It suggests what kind of data is inside, and your operating system uses it to pick which app opens the file when you double-click.
Process running program: A live, running copy of a program. The program is the file sitting on your disk; the process is what exists in memory while it's actually working. One program file can have many processes running from it at once — two browser windows are one program, two processes.

Questions people ask

You double-click a song file. What's actually playing the music?: A music app is. The file is bytes sitting on a disk and can't do anything on its own — your operating system looks at what kind of file it is, finds an app that reads that kind, and hands it over. This is worth internalising because it explains the rest: a file is never doing something, something is always doing it to the file.
I have two browser windows open. Is that one program or two?: One program file on your disk, two processes in memory. Opening it twice loaded the same instructions twice; nothing was duplicated on disk. That distinction explains why your machine slows down when you 'only have one app open', and why closing a window doesn't always quit the app — on Mac you have to Quit it to stop the running copy.
If I rename `song.mp3` to `song.pdf`, does it become a PDF?: No. Nothing at all happens to the contents — you've changed the label and lied to the computer about what's inside. A PDF reader will fail on it because the bytes are still an mp3. Extensions are hints, not converters; actually converting a file needs an app that reads one format and writes the other.
Where does a file I dragged onto my desktop actually live?: In a perfectly ordinary folder called Desktop inside your home folder. The desktop isn't a special area outside the file system — the icons you see are that folder's contents painted onto your wallpaper. Which means you can open it in Claude exactly like any other folder.