Traditionally a Pokemon will either have one evolution or two, with the except of Pokemon such as Eevee which have multiple evolution paths. Although these people can evolve into more than one form, they have never really branched out past that; however, with the mega evolutions now in place, Pokemon can now advance onto yet another stage.
By holding a special stone in battle, some Pokemon will be able to transform into a "mega" version of their final evolution. As you may recall, quite awhile back a new Mewtwo form was announced; however it seems this form isn't a form at all, but rather "MegaMewtwo." When Mewtwo holds onto this special item, he will transform in battle, gain some new abilities, and have stat increases. Although this evolution takes place during battle, the effects only remain up until the fight ends, and that will result in the Pokemon reverting to its original evolution once it is over.
Although we really don't know how many people will feature this "mega evolution" stage, Nintendo has released a new trailer to show off just a few of them. Check it out below: