When you see an online ad that somehow relates to the content you're browsing, you've been targeted by a Contextual Network. The basic concept behind this kind of targeting is that a person searching on a given topic, say new automobiles, is trying to learn about new automobiles and will be highly receptive to advertising that contains information about new automobiles. Technology varies, but generally revolves around keyword matching and some degree of content indexing and/or analysis, with variations ranging from fully automated site/page selection to fully human editorial judgment. Granularity varies from the entire site to individual web pages.

However implemented, the idea is that contextual ads won't be considered intrusive or annoying, but useful and valuable. What’s more, evidence shows that such ads and advertisers tend to be appreciated, and remembered far longer than equivalent but contextually-unrelated ads.

Examples include:
24/7 Real Media
Addynamix
Adtegrity
Advertising.com
AdOn Network
Blue Lithium
BURST Media
Casale Media
ContextWeb
Federated Media
Kanoodle
Right Media
SpecificMEDIA, Inc.
Undertone Networks
ValueClick Media/Fastclick
Vendare/Traffic Marketplace