a "doing business as" or a "d/b/a" is basically an alias. This allows a company or a person to operate under an assumed name. I am the CEO of an assumed name company. Althought we are small we do use something other than the legal name of our owners.
Most often you will see this when a company is a subsidary or was recently aqcuired by another company. Is a long time established local bank which handled all its own loans was recently acquired by a larger bank which now has a "d/b/a" so that it can handle the business of the old bank while running operations under thier new name.
I hope that helps.
Thanks,
Roy