SNMP, at its core, allows a management station to treat its network as a distributed database of health and configuration information (and much more). It's an IETF standardized protocol and "should" work the same way across every implementation. Even the data that can be retrieved and manipulated via SNMP is standardized in TUT:MIBs so conversing with one host about, for example, its routing table can be done in the same way regardless of which vendor created the device being spoken to.
SNMP stands for "Simple Network Management Protocol". The protocol is "simple" (but not necessarily its implementation) as it contains only a few important operations. These operations are functionally:
|GET||Retrieve data from a network node|
|GETNEXT|| Retrieve the next element from a network node|
|SET||Send configuration or control commands to a network node|
|TRAP||A network node can send a notification to the management station|
|INFORM||An acknowledged trap (network nodes can try and send it again if no acknowledgement is received)|