Summons are beings that are magically called out and aid Sora, they are obtained by givng summon stones to Fairy God mother in KH1.
In the Kingdom Hearts series all summons have been Disney character (with the exception of Cloud in KH:CoM). In Kingdom Hearts 1 the summon menu is located within the magic menu but in Kingdom Hearts 2 it was moved on to the command menu because of the two menu thing.