The following airlines serve the following terminals at BWI:

Terminal A is closed for reconstruction but used to house United and Midwest Express.

Terminal B is all Southwest

Terminal C is American, Delta, Northwest and a few Southwest flights (using the old Continental gates).

Terminal D is AirTran, American (old TWA gate), Continental, Frontier, Midwest Express, United, USAirways and what is left of USAirways Express.

Terminal E is Air Jamaica, America West, Air Canada Jazz, British Airways, Icelandair, AMC, charters, and an African carrier.

I know way too much about my least favorite DC airport!