Character advancement in Dungeons and Dragons is done by accumulating experience points. Once you gain enough points you go up a level, which is done by applying another template based on the class you choose to advance, usually the same you started with though many campaigns allow you to take more than one class if you want to. Multi-classing is an important tool in creating unusual concepts, but I'm not going to need to do that with either of the two versions I did here.
Pure Wizards of the Coast Content Only
There is also a defined end state. When you get to a total of 20 levels you are no longer capable of advancing any of your classes. There are some optional rules about getting some specific "Epic Boons" but your character somehow mysteriously stops being capable of learning new things about their classes. While I can sort of see why they went this way, creating challenges for experienced players becomes progressively more difficult if every improvement involves vertical advancement and there is no upper limit, and getting to level 20 is unlikely enough, I still find this facet presents a psychological impact for my enjoyment of character creation.
Character Creation and the Gorgon Archer