Angel financing typically refers to a form of venture capital, which funds businesses at a more 'early stage' than venture capital. Angel investors are also typically individuals (as opposed to venture capital which are formal funds comprising of more than one investor) who have some knowledge or experience in the types of businesses or sectors they invest in. They would invest in a business that finds it difficult to raise more formal venture capital because the business is very conceptual, or have no proof of concept, or very difficult to prove commercialisation, hence the term 'angel' 

http://en.wikipedia.org/wiki/Angel_investor

http://en.wikipedia.org/wiki/Venture_capital