The main difference between Fannie Mae (FNMA; Federal National
Mortgage Association) and Freddie Mac (FHLMC; Federal Home Loan
Mortgage Corporation) is that Fannie May primarily buys mortgages
issued by banks and Freddie Mac primarily buys mortgages issued by
thrifts.
A secondary difference between the two is that Fannie Mae
started in 1938 as part of the "New Deal" and Freddie Mac started
in 1970 in order to create competition in the secondary mortgage
market.