Opcode definitions?

Is there any reliable source where I can read what the opcodes in ollydbg do? For example I know JNZ = "Jump if not zero" (or somewhere along those lines) is there anywhere I can read about all of them that I am coming across? I have tried googling my results of the ones I do not know, but when I google asking what XOR or EAX means I get a bunch of irrelevant topics. Thanks guys, the more you help me, the more I'll be able to help you when I start releasing hacks.