Cambridge has very good public transportation, with service by both the "T" subway system as well as a number of bus lines.

The Red Line of the T connects Cambridge to Boston and a range of other communities in the area.  There are five Red Line stops in Cambridge:  Kendall Square (near MIT), Central Sqaure (an up and coming neighborhood), Harvard Square (home to the famous university of that name), Porter Sqaure (a more residential area with good shops and restaurants) and Alewife, which offers a large parking facility and is a great place to leave the car if you are coming in from the suburbs and don't want to drive.

There are also a number of bus lines that run through Cambridge and connect it to other cities and towns.  A route map and schedule can be found here.