There are few people I know who can authoritatively write about the difference between improv and comedy. John Kinde has just the right skill set, and a great article to prove it. John has an interesting background, and it includes teaching improv. Right now he's living in Las Vegas, but for several years he lived and taught in Santa Maria. One of his venues for learning was here at the Santa Barbara Improv Workshop. For several years he drove down every Wednesday night. He also put on amazing improv shows in Santa Maria and always invited the Santa Barbara players to participate. If you ever get the chance to take one of his workshops, I recommend it.
He's also a public speaker, gives workshops on using humor in public presentations, and writes the blog Humor Power where he shares his knowledge and experiences. I read his writings regularly. Today John posted an article discussing the differences between comedy and improv, and it's great read. I think he nailed the differences and also talked about what makes improv funny.