SNMP is a protocol used to monitor network devices and their performance. It consists of three main components: agents in network devices that collect data, a management information base (MIB) database to store that data, and network management stations that can access the MIB to monitor or control devices. There are three versions of SNMP - version 1 was the initial implementation, version 2 improved security and performance, and version 3 added authentication, privacy and access control to address issues with earlier versions.