There's an instruction set known as x86, which started with 386. All x86 processors support this instruction set, but many also have extensions to these.
For example, the Pentium 4 has the SSE and SSE2 floating point instruction sets.
All of the following are "x86" processors:
There are others, of course, but that should give you an idea.
Itanium 2 uses a completely different instruction set, from what I understand.
x86_64 (aka, AMD 64) supports both the x86 instruction set and an extended 64 bit set.