As your character and followers gain experience points while they fight countless darkspawn, they will most likely level up after certain experience points are reached. The characters that you don't even use will also level up but not as much as your level. Keep in mind that Dragon Age: Origins offers a level 20 cap and hopefully down the road, this cap will increase as more DLCs is released.
You might be wondering how much XP do I need to reach a level? Well, here is the list that you should take a look: