I hope everyone understands, there isn't any perfect definition... Many angels I used to work with have become Super Angels or VCs, or some hybrid of the two.
There's something awesome about someone putting their own money into your business, though... Nothing beats the focus and passion of an investor who's got his own money at stake, and the carrot is huge for them!
Great article in the NYT yesterday about some of the pressures on VCs (note, this is about people who invest in VCs, less about VCs investing in companies!):
http://www.nytimes.com/2011/08/10/te...-homework.html
-j