What is the difference between UNICOM and CTAF?
Essentially nothing. They are both used to announce your position / intentions. There may be some technical difference but in reality its the same (Unicom may be reserved for non airfield ops, like VFR corridors and the like).
Why do some airports have both a TWR and UNICOM frequency?
The UNICOM or CTAF frequency becomes active when the tower closes. Unlike the UK, you can still turn up at a large airport when it is closed, and it is treated like a "Radio" field in the UK. I landed at LGB 2am one morning, I had 5 runways, approach lighting system all at my disposal, and no one else there
Cheers