Explanation: Each process has three IDs: RUID(real user ID), EUID(effective user ID) and SUID(saved user ID). The idea is that a process can temporarily acquire privileges, then give them up when it no longer needs them, and get them back when it needs them again. There is a similar mechanism for groups, with RGID(real group ID), EGID(effective user ID), SGID(saved group ID) and additional groups.

A program that needs to perform certain actions with root privileges normally runs with its EUID, but it calls the seteuid() method to set its EUID to 0 before performing the action requiring privileges.