One of our team was recently asked to give a simple overview of interest rate swaps and how they work. Below was the explanation we put together, using a comparison between an interest rate swap to a fixedrate bank loan to illustrate the key characteristics.
We think it is a nice, concise and clear way to explain interest rate swaps so thought we would share it. Comments welcome.
Bank fixedrate term loan 
Interest rate swap 

Bank loan type  Separate fixedrate term loan  Borrow floating rate (i.e. 90day rate resets at market rates plus lending margin). The bank can normally change the lending margin on an annual review of the facilities. 
Amount being fixed  No flexibility, the full loan amount is fixed.  Interest rate swap contract can be entered for any amount, in multiples of $1m. E.g. Borrowing facility of $10m, decide to fix 50% now, therefore enter a $5m swap. Later on the percentage fixed can be increased by doing another swap contract. 
Fixing of interest rate  Per loan documentation, includes bank lending rate for term (say 5 years) plus bank lending margin = all up fixed interest rate that does not change over the term of the loan  Interest rate fixed by entering an “interest rate swap” contract. The borrower pays fixed rate and receives floating rate under the swap contract. The floating interest rate received under the swap for the next 90 days nets off against the 90 day interest rate paid on the physical floating rate loan above. Net result is an all up fixed interest rate, being the fixed swap rate plus the normal bank lending margin on the borrowing facility. 
Flexibility  Cannot unwind early or unknown penalties applied by the bank for early termination.  At any time the swap can be unwound or closed down. If term swap rates subsequently increase, the swap is closed down at a realised cash gain – being the difference between the contracted swap rate and the higher market swap rate for the term left to run (and vice versa). 
Documentation  Normal bank loan documents  Interest rate swap is a separate legal document under standard “ISDA” bank terms. 
Term of fixing  Interest rate is fixed for the term of the loan  A fixed rate swap can be for any term, does not have to be the same maturity date as the underlying bank loan facility. May be shorter or longer. 
Use of bank credit limits  Loan principal plus 12 months interest cost usually.  Loan principal plus 90 days interest cost, plus credit usage of swap agreement (normally 4% x number of years of swap x principal amount). In addition, if market swap rates subsequently reduce to below the contracted fixed rate of the swap, the bank will add on the unrealised “markedtomarket” revaluation loss onto the total credit usage. The bank normally imposes a a maximum term for swap contracts. They may allow fixing the swap interest rate for 10 years with a “right to break” clause that allows the bank to close down the swap after 5 years if they don’t like the borrower’s credit any more. 
Cashflow  Interest paid monthly or quarterly.  Interest on 90day physical borrowing paid every 90 days and then the bank calculates the difference between the swap fixed rate and market floating rate every 90 days, with the borrower paying the cash difference between the two interest rates to the bank and vice versa. 
Fixing the interest rate in advance of loan drawdown  Not really possible.  “Forward start” swaps can be entered with the fixed rate commencing from a predetermined date. An option can be purchased to enter a fixed rate swap at a future date as well (“swaption”). 