What is all the hoo-haa with Brett Favre?

Excuse my stupidity, but I have little football knowledge. I heard on the radio this morning that Greenbay is offering him $20 mil NOT to play. Didn't he retire?? OK, so he's coming back and has a contract with Greenbay. Why isn't he at camp, and why is there all this hullabaloo? Football folks, will you explain to me this biz? Thank ya!