His contract was up and NBC didn't want him to move to a rival network so they gave him a new contract and a new time for a show similar to his old show. He never retired. He is going to focus more on monologue and skits, and less on celebrity guests. SHOOTER Z is correct, Conan O'Brien took over The Tonight Show.
I've always been a fan of Jay Leno. He's got one heck of a work ethic - often working 7 days a week. He's very down to earth too. He came to Milwaukee not too long ago and I got to meet him. He actually stayed and talked with me for about 15 minutes about cars and football.