Fannie Mae, Freddie Mac, FHA, and VA Mortgage loans?

Can anyone explain in laymans terms the purpose of Fannie Mae and Freddie Mac and what they actually do? I know with an FHA loan, the FHA backs the loan and you get different deals as far as interest rate and down payment (but you may get reamed with PMI or MIP payments). Same goes for the VA backed loans, but you have to be military eligible. What about Fannie Mae and Freddie Mac? All websites say the same old thing "They are Government Sponsored Entities" blah blah blah. Whatever. Simply, what's their purpose? Also, are loans (affected, backed, touched, given?) by them conventional mortgages?

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