There are many different types of interest rate deals that a mortgage lender might offer. The main two are fixed and variable.
With a fixed rate, the rate of interest is worked out at the start of the mortgage and is fixed for a certain amount of time, usually between two to five years.
With a variable rate, the rate of interest moves up and down in line with a specific measure, for example the Bank of England base lending rate, or even at the lender's discretion.