In The Fires of a Red-Hot Reunion!, Ash and his friends visited a Kanto-based festival, where one of the presentations featured the local starter Pokémon, involving a live action show with a Charmander. This made Ash remember his own Charmander, prompting him to tell the story how he got his Charmander, and how it eventually became Charizard and won many battles with him, eventually leaving for training. Interested by the idea of seeing his old friend again, Ash called Professor Oak, who, in exchange for Ash's Unfezant, sent Charizard over to him.
Ya that pretty much explains it :P
Source: Bulbapedia page on Ash's Charizard