Baltimore has a beltway around it, which is called 695. It connects to 95 in two places, the northeast corner and the southwest corner. If you have the misfortune of departing the airport or train station anywhere from 6:30-9:30 AM or 3-7 PM and want to traverse any of these main arteries, be prepared for gridlock and omnipresent, neverending road construction that is not lifted during rush hour. Try to schedule flights arriving during the middle of the day, if you can. 

One option you have coming from BWI airport is to catch the Light Rail northbound only at Terminal 5.  The Light Rail stops at multiple stops in the greater Baltimore area, including downtown.  If you wish to connect with the Baltimore Metro, take the Light Rail to the Cultural Center rail stop.  Get off at Cultural Center, the Metro is a two block walk away.   Exit the station, cross Howard St, continue on W Preston for two blocks.  The State Center metro station is at the intersection of Howard St and W Eutaw (not recommended for the unfit or those with a lot of luggage.) To drive out of the airport, use either 295 North to 695, or 295 to 95 if you're going into Baltimore City itself If you have to take a taxi, empty your bank account because it's not cheap...

Rush hour traffic in the Baltimore area is not nearly as bad as Washington or other major cities: at the worst times allow half an hour from downtown Baltimore to BWI, and take I-95 during the current construction. Taxis are about $25 to $30 to downtown.

If you are not traveling by air or by car (or cruise ship) you can also reach Baltimore by train.  Amtrak services Baltimore on both the Northeast Regional and the Acela Express.   If you are coming from nearby Washington D.C. check out the commuter MARC trains as well, though they only run Monday through Friday.  From Washington a one-way MARC ticket costs $7 whereas you will pay up to $42 for an Amtrak ticket (winter 2009 prices).   The MARC train will stop at New Carrollton as well as BWI and downtown Penn Station - some trains will stop at additional stations as well, such as Odenton.

Once at Baltimore's Penn Station, follow the signs for Light Rail (past the Cafe).   The Light Rail station is located down a flight of stairs with a ticket machine under the stairs.  Penn Station is a branch line of the Light Rail, which will connect with the main Light Rail system at the University of Baltimore station, or you can continue down to the Inner Harbor area via Camden. 

DC Metro residents and tourists can use the DC Metro SmartTrip card to travel from anywhere in the DC Metro area to anywhere in the Baltimore Metro area.  Take the DC Metro to the Greenbelt Station.  From there take Metrobus B30 to Baltimore Washington Airport.  Get off at the second stop (Terminal 5) (the bus driver will call the Light Rail stop out). Take the Light Rail downtown.  The ticket machines are just inside the terminal.