This is the London Inter Bank Offer Rate. It is the rate that the banks use to buy and sell money to each other. Most mortgages which track this rate change the rate quarterly, using the three month LIBOR. As for rates which track the base rate, lenders offer deals at a percentage above the LIBOR rate. They may also offer deals which depend on foreign currency rates, for instance US dollars, which have had much lower rates than the British Pound. This is a more volatile rate than the base rate, so can be riskier.