Quote from rew:
Here is what an interest rate swap is:
Harry borrows $10,000,000 from Sally at a fixed rate of 3%.
Sally borrows $10,000,000 from Harry at a variable rate of 3 month LIBOR + 2%.
They agree to pay the principal down according to some fixed schedule. Of course, since they owe each other the same amount of principal no principal actually changes hands. But the interest payments do. The contract involves virtual principal and real interest payments. If rates go up Harry wins. If rates go down Sally wins.
BTW, the notional amount of this particular derivative is the principal balance, a virtual number. When folks talk about the trillions of dollars of derivatives floating about, they're adding up these notional amounts, even though they don't actually represent an amount that anybody owes to anybody else. (The interest streams are real obligations, but not the principal.)