It is often some form of LIBOR rate, but it can take many forms, such as a consumer price index, a house price index or an unemployment rate.
The rates are calculated by an independent organisation, such as the British Bankers Association (BBA) as the average of the rates quoted by a large panel of banks, to ensure independence.
The ISDAfix rates used are calculated daily for an independent organisation, the International Swaps and Derivatives Association, from quotes from a large panel of banks.
In the credit derivative market a similar concept to reference rates is used.
Typically the benchmark LIBOR is the three-month rate.