A social entrepreneur is someone who uses their business skills to produce a financially sustainable, socially beneficial venture.
As a matter of fact, I run a training program for Social Entrepreneurs at the college level. It's called Compass Partners, and you can check it out at www.compasspartners.org.
Some other great resources are Ashoka, Social Edge, The Unreasonable Institute, Sparkseed.
It's a huge movement in business right now and is really getting some market share and press. I'd love to chat if you're interested in this!