The Theotokos is the Virgin Mary, the mother of Jesus Christ. Theotokos is distinct from Christotokos although both words refer to the Virgin Mary. Theotokos refers to the the person who gave birth to God and Christotokos means the woman who gave birth to Jesus.
Theo refers to "god" and "tokos" is the giving birth part. Like the spear-bearer or doryphoros, theophoros literally means god-bearer, although Theotokos is also often translated as god-bearer.
Those who supported the Nestorian heresy considered Mary the Christotokos rather than the Theotokos, and believed that she bore Jesus in human form. Those who consider Mary the Theotokos believe she bore the divine Jesus.