1. A method comprising: using one or more computers, obtaining a first set of information comprising, for each of multiple publisher sites, a set of publisher site tags, and for each of multiple advertisements, a set of advertisement tags; using one or more computers, in selection of an advertisement, of the multiple advertisements, to be served in connection with a serving opportunity, wherein the serving opportunity relates to an advertisement serving opportunity on a publisher site of the multiple publisher sites, utilizing, as a factor in the selection, matching based at least in part on one or more publisher site tags of the publisher site and one or more advertisement tags of the advertisement; and using one or more computers, facilitating serving of the selected advertisement in connection with the serving opportunity.