SNMP TRAP terms

For V1, there is the simple TRAP, which is sent from SNMP Agent to SNMP managers/trap receivers

From V2c on, including V3,

• TRAP is renamed to NOTIFICATION,
  
• NEW: INFORM -- essentially ACKed-trap

V3: added security. Both UserID and EngineID are needed for security purpose.

• For TRAP/NOTIFICATION, the engineID is the AGENT's engineID, which has to be added to the userlist on the trap receiver side.
• For INFORM, the engineID is the receiver's enginerID, which can be auto discovered in most cases.

See the NET SNMP V3 tutorial for more details. http://www.net-snmp.org/tutorial/tutorial-5/commands/snmptrap-v3.html