Fannie Mae:

Is a government sponsored enterprise (GSE) and put in charge by congress with the intent to maximize borrower’s cash flow, and provide stability and access to the mortgage market in the United States.

Fannie Mae operates in the secondary market of mortgages in the United States.  Instead of providing direct loans to the consumer, Fannie Mae works with banks, and other financial institutions in the mortgage market to secure the lender has sufficient cash flow to lend at a rate of market value.

Freddie Mac:

What makes the instrument that is the financial institutions work?  The families that pay there mortgage every month.  When a family acquires a new home, there monthly payments (a combination of the principal and interest) is transferred to the financial institution and its investors.  Managing risk, also know as “credit risk”, is what generates income for Freddie Mac.