What is SNMP and SNMP Trap?

First lets see what SNMP stands for. It's Simple Network Management Protocol. It is developed with the intention to manage growing elements in the internet. Why it is called Simple is that, it has only five types of messages. And the other fact is that it can rely on a connection-less or unsupervised network link.

SNMP is based on the Manager and Agent model. So that it's Manager provides interface between a human Network Manager and the Managing System while the Agent provides interface between the Manager and  Physical Devices which are being managed. 

SNMP trap is one of the five message types being used in SNMP protocol. The five types of messages that the SNMP trap is using are as follows.

1. GET
2. GET-NEXT
3. GET RESPONSE
4. SET
5. TRAP

GET and GET-NEXT messages are sent by an SNMP Manager to the Agent. Upon the arrival of those messages, the Agent will send a GET RESPONSE message to the manager. SET is for requesting variable changes remotely. 

And at last comes the TRAP. This message type is quite different from the other messages. This message will allow an Agent to send messages to SNMP Manager spontaneously. Usually an SNMP trap is used by an Agent to aleart their Manager about a problem that they experienced.

Following diagram simply illustrates how an SNMP Trap works. 

Image Courtesy : http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094aa5.shtml

More information on SNMP Trap can be obtained from the following link:
http://www.dpstele.com/dpsnews/techinfo/snmp/snmp_trap_basics.php