Boston's transport consists of the country's oldest subway system, buses, trolley buses, ferries around the harbour and the commuter rail. For getting around the Boston-Cambridge area the subway, or the 'T' as it is known, is the best as it is easy to use, cheap, fast and safe. It serves most of the city and on the whole visitors will have little use for the bus network, which is cheaper but more confusing for newcomers. Both operate from about 5am to 12.30am, but a 'Night Owl' bus service has been introduced to provide transport along main bus routes and parallel to subway lines on Friday and Saturday nights until 2.30am. Different types of transport require different tokens, but various passes are available for unlimited travel on buses and the 'T'. Taxis are plentiful but expensive, although water taxis are a novel way to explore the city. Licensed cabs are best found at cabstands usually near hotels or can be reserved by phone; they are metered and provide receipts. Boston is a motorist's nightmare and there is no need to rent a car while in the city, unless planning excursions. Rental agencies require drivers to be 21 years old with additional surcharges for under-25s.
Navigating Boston's streets is very hard if you are not familiar with the area. Driving is to be avoided if possible. While other cities have their streets laid out in a grid (New York, Chicago, Indianapolis) or along a river, lake, or other geographical feature (New Orleans, Cleveland), the streets of Boston are essentially paved versions of the 17th-century cattle trails and dirt roads they replaced. There are many one-way streets, usually arranged haphazardly and poorly marked for drivers. Signage is nothing short of terrible and often you will have no clue what street you are crossing. Bostonians have adapted to this lax signage by — in many cases — completely ignoring street names outside their immediate neighborhoods. Most navigate by landmark and memory. Watch out for lots of double-parked vehicles. Boston's drivers, pedestrians, and bicyclists are notorious for being aggressive. Especially avoid driving during rush hour on weekdays; streets and highways become extremely crowded (the downtown population doubles each workday). Walking, especially downtown, is the best way to go.
For most tourist destinations in Boston and Cambridge, you should leave your car behind and take the subway. You'll do a bit more walking, but that will give you a chance to see the sights. However, much of the parking in Boston and surrounding towns is limited to neighborhood residents, who have stickers identifying their cars. You should therefore check whether parking on a particular street is open to you, and consider using metered parking or public or private lots.
If you are unfamiliar with the area and don't plan on going beyond the areas covered by the subway then it is best to avoid a car. You will save yourself a lot of money and aggravation.