What are the legal issues I have to worry about if I want to sell photos that I shoot in public places?

Please post a link to where I can read about the legality of selling my photos. I imagine I can take a photo of a large crowd and sell it, but there are probably legal issues if I shoot a photo of a cute kid or a hot chick. You can give your two cents if you like, but I would prefer a link to where I can get info about what types of public photos can be sold. Thank you Yahoo! Answers!