A company is said to be "doing business as" when the name under which they operate their business differs from its legal, registered name. Some states require dba or fictitious business name filings to be made for the protection of consumers conducting business with the entity.
If you're starting a sole proprietorship or a partnership, you have the option of choosing a business name or dba ("doing business as") for your business. If you want to operate your business under a name other than your own (for instance, Carol Axelrod doing business as "Darling Donut Shoppe"), you may be required by the county, city or state to register your fictitious name. (Note: No fictitious business name may include the words "corporation," "Inc.," "incorporation" or "Corp." unless it's a corporation registered with the Secretary of State.)
Procedures for filing for a fictitious name vary among states. In many states, all you have to do is go to the county offices and pay a registration fee to the county clerk. In other states, you also have to place a fictitious name ad in a local newspaper for a certain amount of time. The cost of filing a fictitious name notice ranges from $10 to $100. Your local bank may also require a fictitious name certificate to open a business account for you; if so, they can tell you where to go to register.
In most states, corporations don't have to file fictitious business names unless the corporations do business under names other than their own. Incorporation documents have the same effect for corporate businesses as fictitious name filings do for sole proprietorships and partnerships.