word choice - How does an "agency" differ from a "company"?

When should a company be called an agency?
Typically advertising and recruitment companies call themselves agencies, is there a reason other than tradition for this?

Answer

An agency is a company that acts on behalf of another company — as an agent. For example, an advertising agency has clients for whom it sells advertising. McDonald's sells fast food to the public, so it is merely a company. It has an advertising agency, however, to create its commercials.

Similarly, a recruitment agency provides a service for employers, and so on.