Summons are beings that are magically called out and aid Sora, they are obtained by givng summon stones to Fairy Godmother in Kingdom Hearts. In Kingdom Hearts II, Summons give Sora their very own amulet which he can use to summon them. All summons are also the victims of the Heartless' quest to destroy worlds. As explained by the Fairy Godmother, the summoning gems are the person's crystalized bieng that have not fallen victim to the darkness and destruction of thier worlds, and are able to be called into battle for a limted time.
In the Kingdom Hearts series all summons have been Disney character (with the exception of Cloud in KH:CoM). In Kingdom Hearts the summon menu is located within the magic menu but in Kingdom Hearts II it was moved on to the command menu because of the inclusion of a second menu.
List of Summons
Kingdom Hearts II