Interest rate swaps take many forms. Typically they consist of a number of component swaps on a frequent basis according to a predetermined payment schedule.
Usually, one leg involves quantities that are known in advance, known as the "fixed leg", the other involves quantities that are not known in advance, known as the "floating leg". The floating leg must therefore be reset against an agreed reference rate, which will become known at some point before the payment or settlement takes place. Ideally, the determination of the reference rate must be outside the control of the counterparties, otherwise a conflict of interest will arise. However, many financial products in the retail market (such as capped mortgages) involve reference to a managed interest rate which is actually controlled by the mortgage provider. Typically, the reference rate is some figure made publicly available by a third party information vendor, or by government agencies. For example, BBA LIBOR.
Once a component of the floating leg is fixed (or "reset"), the fixed and floating components can be swapped or settlement (typically one or two days after the fixing date).
The present value of a vanilla swap can easily be computed using standard methods of determining the present value of the components.
References
See also