What are Expansion slots?
Earlier motherboards rarely offered additional functionality besides offering a place for the CPU and RAM to be plugged in. The capability of the system had to be "expanded" with the use of external cards that carried the additional circuitry to fulfil the function. Additional circuitry usually included hard disk controllers, display adapters, sound system, etc., all of which are nowadays present "onboard" - referring to the inclusion of all this circuitry on the motherboard. Today, expansion slots on motherboards are mostly PCI, PCIE, and AGP. A couple of rarely-used slots are CNR (Communications and Networking Riser) and AMR (Audio Modem Riser). These special slots were designed to incorporate the interfaces for networking and audio devices; they did not contain the processing circuitry, and merely acted as a channel. The processing was done by the host CPU.