SIP is a protocol for initiating, managing, and terminating multimedia sessions over the Internet. It allows users to establish multimedia sessions including voice, video, and text. SIP components include user agents, proxy servers, redirect servers, and registrars. SIP uses request methods like INVITE, ACK, BYE and response codes to set up and terminate sessions between user agents. SIP addresses users in a globally reachable way and allows real-time communication between parties.