There are two sides to online advertising, a legitimate one and an illegitimate one. The legitimate side of online advertising includes search engine advertising, advertising networks and opt-in e-mail advertising. The illegitimate side is dominated by spamming.

Though the range of advertising options has expanded since in the commercialization of the Internet, the use of rich media and static images is extremely popular. The ever-increasing audience of online users will likely continue to be a major advertising market.

A significant number of firms, from small businesses to multinational corporations, incorporate online advertising into their marketing strategy. This is even true of firms which conduct their business through more traditional brick and mortar channels. In response to this demand, a number of firms specialize in facilitating online marketing. Therefore, online advertisements typically involve at least two separate firms: the advertiser or agency which purchases or sponsors the advertisement and the publisher or network which distributes the ad for display. Additional parties may also be included, such as an ad serving technology provider, a third party sales network, or other combinations.

In capitalizing on the increasing importance of the Internet as a marketing medium, the online advertising industry has developed specialized technical systems to manage the ways ads are distributed and viewership totaled. The Internet Advertising Bureau (IAB) has established guidelines for the counting methodology, size requirements, and other aspects of the business.

Because of the close relation between technical innovation and online advertising, many firms specialize in both. For example, most search engines couple their search service with an advertising program, exploiting the benefits of keyword-based search technology by including ads in search results. Many technology firms specialize in ad serving, the systems used to select the ads to show, optimize results, and generate reports.