An angel investor is an individual who provides cash, and sometimes assistance as well, to young companies run by people the angels don’t initially know well – that is, they aren’t “friends and family.” In the US they are usually accredited investors, but not always.
Often angels invest on their own, but in the US and globally more formal angel investor groups as well as equity crowdfunding sites enable angels to collaborate and invest together.
Depending on the circumstances an angel investor might then become an informal advisor to the company, or sit on the board of directors, or be inactive (a “passive investor”).
In the US the typical angel investment is $25,000 per investor per round per company, but it can be more or less. And the investment is typically equity although some angels do invest debt in companies unable to secure it through other means.