The choice you have to make is whether or not to go "prestige".

If you do...you give up all you have earned...well, not all. You keep your kills, points, etc but lose all your weapons and achievments. You basically start over. As you level up, the icons for each level are different from the first round.

I've been at level 55 for a while now. I'm not ready to give it all up yet and I'm still trying to complete challenges so I'll probably be here for a bit.