What are Angel Investors?

I read a discussion yesterday in regards to a seed money to start up a business and it seemed very interesting for me but, I have questions: What an angel investor means? How can they be found? What can they expect? I have a brand new business operating for 2 years but due to the present economical situation I have not been able to expand as long as we can not obtain a loan to do so. We have still 50% more capacity. This is a very specialized business with a good margin where there is sales volume. Can somebody provide any information? Thanks for your help.