UID - USER ID

Each Unix-Like operating system user has a integer identification number associated with their login. This is known as the UID or User ID.

Software can manipulate integers extremely rapidly. Strings of characters take longer to evaluate and compare. Integers require less memory and disk space to store than character strings.

For these reasons, Unix-like operating systems map your login to an integer and use the integer for all internal functions relating to users. Things like file ownerships are stored as an integer value. A file, /etc/passwd, is used to map user logins to integer values and vice versa. Sometimes these values are databased for faster lookup.

If you remember the old Johnny Rivers song, Secret Agent Man (1966), and specifically the lyrics, "given you a number, and takin' way your name...", well that's what Unix does internally. Externally, you get to keep your name and most programs that provide user information will convert the numbers back to a login for display.