Drummer street Bus Station is the main bus station in Cambridge and is located in the city centre. Most local, national and international bus services terminate at Drummer street, but not all of them. Priority is given to local bus services at the bus station so some national coach services arrive and depart from bays on Parkside street next to Parker's Piece park. Services that use bays on Parkside include those to Heathrow and Gatwick airports, London Victoria Coach Station, Oxford and Brighton. There are also a number of bus stops along Emmanuel street (not to be confused with Emmanuel Road!) along the north west side of Parker's Piece park to some local destinations including the park and ride.
Most buses do not take credit card so make sure you have enough cash. The price of a return ticket from Cambridge to Heathrow airport is approximately 35 pounds.
There is one thing that Cambridge is not short of and that is local buses. The majority of the local buses start in the City Centre and depart from either Drummer Street Bus Station or Emmanuel Street with additional stops on St Andrews Street. One route that all visitors should be aware of is the city circle route which is free and has stops all around the key city centre locations. A map of the route can be found here.