You're referring to a banner-exchange program.
Many companies pay a lot of money to advertise their services via banner ads on Web pages.
However, you don't have to have an advertising budget to get some online publicity. It's very common for companies and individuals to barter for ads. In other words, two companies will agree to post each others' ads at no additional cost. While it's possible to hammer out such an agreement by contacting the owner of a particular Web site and working out the details between the two of you, you might want to look into one of the services that specialize in setting up banner-exchange agreements. You can find many more such services by doing a simple search for banner-exchange programs in any of the major search engines.