What is Advertising Agency?

Legal Definition
An advertising agency is a service based on advertising business dedicated to creating, planning, and handling advertising and sometimes other forms of promotion and marketing in a promotion for its clients. An ad agency is generally independent from the client; it may be an internal department or agency that provides an outside point of view to the effort of selling the client's products or services, or an outside firm. An agency can also handle overall marketing and branding strategies and sales promotions for its clients.

Typical ad agency clients include businesses and corporations,non-profit organizations and private agencies. Agencies may be hired to produce television commercials, radio commercials, online advertising, out-of-home advertising, mobile marketing, and AR advertising, as part of an advertising campaign.
-- Wikipedia
Legal Definition
A firm that creates, designs, plans, surveys, and basically helps a company advertise their services or products. They are not agents of the company that hires them but work in their own interest.