In advertising, a network is a group of ads with a particular demographic alignment. Put another way, an ad network is a selection of ads that are targeted to a particular aspect of the population based entirely on their placement.
Look at it like this: make a list of all the bars in an area. Now narrow the list down to bars that cater to African-Americans over the age of 35 with high disposable income, ie, bars in middle-class neighborhoods that feature live jazz. Any advertising in or in the vicinity of those bars that are under an ad agency's control, everything from posters to cocktail napkins, would be considered a network because there's a better chance of those ads being recalled by a given demographic, and companies with that demo as a target audience (in this case, say, makers of whiskeys, mid-range cigars and near-luxury automobiles) can comfortably assume that ads placed around there have a better shot of being absorbed by who they're trying to reach. As a corollary, if the demographic you were trying to reach was still African-American but younger with a proportionally lower median income, your network would focus on clubs with a hip-hop focus, would be more centered outside the establishments and would include local areas of mass transportation, bus shelters and subway stations and the like, because lower income potential customers, particularly those in high density urban centers, are less likely to have their own cars.
Ad networks are reductive things by definition, but people as a whole prefer their advertising to be targeted specifically to them because advertising that resonates with the viewer is less obtrusive than those that don't - to take it to an extreme, ads for feminine hygiene products placed in a men's bathroom won't tank the sales of Tampax because men don't buy them anyway, but would be an waste of money and would probably result in an overall lowered perception of the brand due to Word of Mouth and Spill.
The most interesting thing about networks is the intelligence required to build effective ones - without massive amounts of market research, any foray into this realm would be like firing a shotgun at a black piece of paper in the dark.