From MEPIS Documentation Wiki
Linux is an account-based operating system. This means that every program and process that is run is attached to a single user account. When you log in as a user, you take on that user's credentials, and thus can perform actions which are permitted to that user.
Suppose user warrenw has access to folder /home/shared/coolstuff, but user billg does not. If a program, such as a text editor, is running with billg's credentials, it cannot access /home/shared/coolstuff (to save a file there, for example). If the same program is run with warrenw's credentials, it can save files to /home/shared/coolstuff with no problems.