Washington is, of course, the capital city of the United States. One of the world's best collections of museums is located in the heart of town, along and around the greenspace known as the "National Mall". The museums - and a growing number of monuments - continues to make Washington one of the country's most popular tourist destinations. And the overwhelming influence of the U.S. government makes it a popular spot for conventions and meetings of all kinds.
The Metro system (that's the subway and above-ground rail) is the only way to get around when visiting DC, especially for those who are doing "the tourism thing". There are lots of hotels near Metro stations in DC, northern Virginia and suburban Maryland. From there you can get to all the hot spots in the core of the city by disembarking at the Smithsonian, Union Station, Archives, Capitol South and Federal Triangle Metro stops. Metro uses a "fare card" system - you buy a magnetically-encoded pass that holds a dollar value which gets debited each time you leave the system. Trip costs are based on distance traveled, so check the charts in the station before you buy your card.
Anyone headed to the DC Convention Center will want to find their way to the "Mount Vernon Square-Convention Center" stop on the Yellow and Green lines.