1. An ad server system, comprising: a database that stores ads for providers of product-related services such that at least some of the ads are stored in association with a corresponding product descriptor that describes one or more products to which the respective ad corresponds, the product descriptors being separate from the ads, wherein at least some of the ads are additionally stored within the database in association with ad category descriptors that specify types of product-related services to which such ads correspond; and an ad server comprising computer hardware, the ad server operative to: use the associations between the ads and the product descriptors to select ads from the database, the ads being limited to ads for product-related services associated with a product, use the ad category descriptors to filter out ads falling outside a pre-selected set of service categories for a given web page, and output the ads for display on one or more web pages related to the product.