The software world
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. Why it matters
Part of Files, folders and paths → It is a hint, not a rule. Renaming song.mp3 to song.pdf changes the label and nothing else — the bytes are still an mp3, so no PDF reader will make sense of them. Converting a file needs an app that reads one format and writes the other. Worth knowing too: Finder and File Explorer hide extensions by default, which is why two files can look identically named and behave completely differently.
see also