Difference between x64 and x86. 
It usually refers to x86 for 32 bit OS and x64 for system with 64 bit.
Technically x86 simply refers to a family of processors and the instruction set
they all use. 
The 64 bit computers can run both 32bit programs and 64 bit programs

Stole "Distured Dingos" answer but made it easier to read.