Hi,x86 is actually just notation for a CPU instruction set which has been used in most modern day processors. x86 actually has three different subsets: x86-16 (16-bit), x86-32 (32-bit) and x86-64 (64-bit).
The 32-bit version of x86 became so common that eventually the term "x86" by itself was known to be the 32-bit iteration. The newer 64-bit version can be referred to as it is above, but far more often is simply called "x64", which can understandably be confusing to someone who doesn't have a lot of knowledge concerning computer hardware, as it sounds like a step backwards!
Because of this, the 32-bit version of Vista is "x86", and the 64-bit version is "x64".
Hope this helps.