I live and breathe this stuff, so it catches me off guard when someone asks, "What do you mean by Social Enterprise?" - it's easy to forget it's still not common place.
A Social Enterprise is Business That Exists to Solve a Social Issue
A social enterprise exists because they are tackling a social issue like poverty, homelessness, human trafficking, the environment, mental health, equality...
AND... they have a business model, with revenue that funds a significant portion of their impact. Some common models include:
Hiring: Greyston Bakery (they bakes brownies for Ben & Jerry's & hires those with barriers to employment, such as those who are formerly incarcerated)
Donation: Goodwill, Salvation Army
Volunteer: Wikipedia
Upcycling: TerraCycle
Microfinance: Kiva
Co-operative: Equal Exchange
Buy-one give-one: TOMS Shoes (in the beginning)
Pay-what-you-can: Jon Bon Jovi’s Soul Kitchen
Socially responsible marketing: The Body Shop
They can be either non-profit or for-profit companies - unlike other non-profits, they're not entirely dependent on grants, and have more control over their impact. Unlike other for-profits, their impact is ahead of their profit, which can make growth difficult. So, if you see one, support them, and you'll make the world a better place.