AMQP stands for the Advanced Message Queue Protocol and is a specification for how messaging clients and brokers can interoperate. AMQP is a specification of a wire-level protocol for client to message broker communication. It is not a messaging system like ActiveMQ, but just a messaging protocol.
ActiveMQ now implements AMQP 1.0!
ActiveMQ also supports other open wire protocols:
Through these protocols, ActiveMQ can support clients in C, C++, C#, Ruby, Python, Perl, PHP, Pike etc. AMQP is most similar to OpenWire, because both OpenWire and AMQP are designed for high performance messaging, through a binary (rather than text-based) format. As a text-based format, STOMP is much easier to implement, but gives somewhat slower performance.