Self-promotional ad a company runs on its media outlets to put unsold inventory to use.
House ads arise from unsold advertising inventory. Many ad spaces are relatively fixed, and house ads are often the logical solution to fill the space. House ads are often used to promote new features, other in-house revenue generators, and other media properties.
House Ads Still Taking Up Space Online
ClickZ (June 28, 2000)
House Ads Represent Nearly Twenty Percent of Online Advertising
AdRelevance (June 2000) PDF