Destiny developer Bungie has detailed how you’ll be able to level up beyond the game’s soft level cap of 20.
You can’t grow your characters beyond level 20 by conventional means – collecting experience points. Once you reach 20 you’ll see XP collected into Motes of Light instead.
It’s these Motes of Light which you can then craft into special armour – courtesy of the Speaker character in the game’s Tower social hub.
This armour will boost your character stats higher (the highest shown so far is level 29), although the character menu itself will still show 20 as your upper limit.