x86-64 is also known as AMD64, x64, and Intel 64. It was built by Advanced Micro Devices (AMD) as an extension to the 32-bit x86 architecture. The x86 term refers to an entire family of processors – it began as a 16-bit instruction set for 16-bit processors (x86-16) was extended to 32-bit processors (x86-32) and has now been to extended to 64-bit processors.
64-bit processors provide some significant advantages over 32-bit processors in the area of enhanced memory capacity. 32-bit processors can handle up to 4GB of physical memory, but 64-bit can support up to 32GB. x86-64 processors are backward compatible, so 64-bit computers are able to run both 32-bit and 64-bit programs without any performance degradation.
Linux was the first OS kernel to run the AMD64 architecture in long mode (the intended, primary mode of operation) beginning in 2001 before the hardware was even available for it, and it also provides backward compatibility for 32-bit executables. SUSE Linux Enterprise 15 supports 32-bit runtimes on AMD64 architectures, which may be needed to run tools to set up software or hardware, that are still compiled as 32-bit binaries.