As I write more about the online advertising business, I realize I need a glossary to send people to for certain terms. So here it is. It's incomplete, of course, but will get better over time. Definitions for terms in italics can be found elsewhere in the glossary.
Advertising Inventory: The supply of opportunities to display advertising in a particular medium.
Advertising Network: An organization which buys advertising inventory from a group of publishers and sells it to a group of advertisers and media agencies.
Agency: A company that provides advertising services to an advertiser. These services usually fall into two categories: creative services (originating ideas for campaigns and creating advertising as part of these campaigns) and media services (buying advertising inventory on behalf of the advertiser).
Network: See Advertising Network