What is the difference between a marja and a mujtahid?

1. Mujtahid: This is the title given to anyone that has reached a "complete" level in fiqq. Basically it means that you have a license to have an opinion on it(to say it with humor). At this level you don't need a marja or follow anyone.
2. Marja: A Marja is a Mujtahid that decides to guide others officially. Basically you start writing a Risalah(if you want) and you are open to guide anyone that chooses you to be the wisest.