Motherboards are subcategorized by the type of processor socket they have. The processor socket (also called a CPU socket) is the connector on the motherboard that houses a CPU and forms the electrical interface and contact with the CPU. Processor sockets use a pin grid array (PGA) where pins on the underside of the processor connect to holes in the processor socket. Computers based on the Intel x86 architecture include socket processors.
See “What You Need to Know About Motherboards” in the Quick Reference section of Webopedia.