There are over 21,000 advertising establishments in the United States. About 6 out of 10 write copy and prepare artwork, graphics, and other creative work, and then place the resulting ads in periodicals, newspapers, radio, television, or other advertising media. Within the industry, only these full-service establishments are known as advertising agencies . Many of the largest agencies are international, with a substantial proportion of their revenue coming from abroad. About 3 out of 10 advertising firms specialize in a particular market niche.
Some companies produce and solicit outdoor advertising, such as billboards and electric displays.
Buses, subways, taxis, airports and bus terminals also frequently carry ads. A small number of firms produce aerial advertising, while others distribute circulars, handbills, and free samples. Groups within agencies have been created to serve their clients' electronic advertising needs on the Internet. The Internet is a medium that fosters rapid growth of advertising and commercial activities. Advertisements often link users from one website to the company's or product's website where information such as new product announcements, contests, and product catalogs appear.