Advertising is differentiated from public relations in that an advertiser usually pays for and has control over the message. It is differentiated from personal selling in that the message is nonpersonal, i.e., not directed to a particular individual.
Ad networks can also be divided into first-tier and second-tier networks. First-tier advertising networks have a large number of their own advertisers and publishers, they have high quality traffic, and they serve ads and traffic to second-tier networks. Examples of first-tier networks include the major search engines. Second-tier advertising networks may have some of their own advertisers and publishers, but their main source of revenue comes from syndicating ads from other advertising networks.
While it is common for websites to be categorized into tiers, these can be misleading. While Google is in the clear majority of advertisement impression served, other networks that could be labeled as tier 2 actually dominate over these tier 1 ad networks as far as the number of customers reached...