GSM is a network used by wireless service providers. It stands for Global System for Mobile Communications and its main competition is the CDMA (Code Division Multiple Access) network.
GSM and CDMA used to differ in terms of signal quality and consistency. Eventually both improved to the point where there are very few differences between the two. Still, most cellular service providers only use either GSM or CDMA because it is very expensive to maintain both types of service.
For our purposes, this GSM/CDMA distinction is important because even unlocking your GSM phone will not make it compatible with a CDMA-only network.