Often they're used together.
closed as off-topic by Monica Cellio, Martijn Pieters, random, rene, Shadow Wizard Jul 29 '14 at 18:34
This question appears to be off-topic. The users who voted to close gave this specific reason:
x64 refers to the 64-bit version of the x86 architecture. (also known as x86-64)
64bit refers to 64-bit computer architecture. (not necessarily x86)