A soundcard is a smaller card which usually connects to a PCI or PCI-Express slot on the motherboard of a desktop computer. Sound cards allow for audio to be transmitted from computer data to any output device such as speakers or headphones.
Soundcards typically have an input for a microphone device as well as a 3.5 jack output port for sound. Higher end cards have more inputs and outputs as well different technologies to facilitate for serious audiophiles.
Soundcards typically come in 5.1 or 7.1 compatibilities which allows for 5 speakers and 1 subwoofer to be connected simultaneously or 7 speakers and 1 subwoofer.
USB sound cards are also available which simply plug into any USB port and allow for a 3.5 jack input for mic or output for speaker or headphones. These are particularly useful for laptops where integrated soundcards may not be able to be replaced