Ad inventory is a term that refers to all available ad space on a site. Sometimes, inventory and ad space are used interchangeably.
Premium inventory: This category contains the most-valuable ad inventory, typically from recognizable publishers, high-traffic pages, and highly visible areas of a webpage.
Remnant inventory: This category contains the inventory that wasn’t sold directly to advertisers through direct deals and other primary inventory-monetization channels. Remnant inventory is typically sold for less than the publisher’s standard price.
Long-tail inventory: This category contains ad inventory on low-traffic sites and blogs. The main ad exchange for long-tail inventory is Google AdSense, but publishers who seek better monetization alternatives to AdSense join affiliate programs and other ad networks.