When a computer is used by many people it is usually
necessary to differentiate between the users, for
example, so that their private files can be kept
private. This is important even if the computer can
only be used by a single person at a time, as with
most microcomputers.
Thus, each
user is given a unique username, and that name is used
to log in.
There's more to a user than just a name, however. An account is all the files, resources, and information belonging to one user. The term hints at banks, and in a commercial system each account usually has some money attached to it, and that money vanishes at different speeds depending on how much the user stresses the system. For example, disk space might have a price per megabyte and day, and processing time might have a price per second.