A TRAP is a one-way communication, and is one of two types of notifications that can be delivered over SNMP. An entity that receives a TRAP does not acknowledge that it has received it (like snmptrapd, and thus it is not possible for the sender to be assured that the TRAP successfully made it to the destination. Because of this, the usage of INFORMs are recommended instead.
SNMPv1 actually had a more complex TRAP definition that included a number of parameters.
In SNMPv2c, the TRAPv2 PDU was defined which is significantly simpler and matches the on-the-wire format of the other protocol operations.