@dappled_leaves is right: X86 is the 32 bit OS while X64 is the 64 bit.
I had Windows 7 64 bit as my OS last year, and it used this dual X86/X64 representation. Also, I just checked, and my Windows 8.1 system identifies it as “64 bit operating system, X64 based processor.”
If you have a program files folder with the suffix (x86), it means your windows is 64bit, because 64bit windows creates a separate program files folder for 32bit programs.