x86-64 is a 64-bit version of the x86 processor architecture, which Windows PCs have used for several decades. It is similar to x64, but refers specifically to processors that use the x86 instruction set.
CPUs with the x86-64 architecture run in 64-bit mode by default, but are also backward-compatible with 32-bit and 16-bit applications. In other words, any program that runs on a 32-bit x86 CPU should run on an x86-64 CPU with no emulation required.
The main benefit of an x86-64 processor versus a standard 32-bit x86 CPU is significantly more addressable memory. A 32-bit processor can only reference 232 (4,294,967,296) addressable values in memory — roughly 4 gigabytes. A 64-bit processor supports 264 (18,446,744,073,709,551,616) addressable values. Therefore, an x86-64 CPU does not support double the memory of an x86 CPU, but 4,294,967,296 times more.
Intel and AMD are the two primary manufacturers of X86-64 processors. The two technologies are labeled Intel 64 and AMD64, respectively. AMD released its first 64-bit CPU (Operteron) in 2003, followed shortly by Intel, which released its first 64-bit CPU (Nocona) in 2004. Today, nearly all processors are 64-bit.