Of course, with networks developing so rapidly, these categories are by no means mutually exclusive or set in stone. How you break down the categories depends on your intended outcome. In fact, Michael Katz, president of interCLICK (the fastest-growing ad network, according to comScore), says, "I think you could make the argument that some networks are segmented by the pricing they offer, like CPA versus CPM versus CPC, but that's not really a determining feature. I think it's more accurate to remove geo-networks and the pricing model, and add in... transparent versus blind networks, and premium versus remnant networks, along with possibly even contextual networks."

Katz suggests advertisers use networks to find the right balance between granular targeting and run-of-network reach, building brand awareness and generating immediate response. For example, reaching in-market buyers -- in categories like automotive, financial services, travel and retail and shopping -- via behavioral targeting can be tremendously beneficial to a campaign. It enables direct response message delivery to an exact core audience, which can then be supported by a branded push using rich media like video. The specifics, of course, will depend on your product, your campaign goals and other parameters.

Susana Tsui, regional managing director of Neo@Ogilvy, a division of OgilvyOne Worldwide, says, "Although every campaign should have a media mix, larger brand advertisers would not usually rely exclusively on ad networks. However, some SOHO [small office, home office] businesses that have different requirements than brand marketing may benefit greatly from network buys, which tend to be very performance based and can work very well for direct response."