Ad blocker
Software on a user's browser which prevents advertisements from being displayed (Source: IAB)

Ad network
Organization that aggregates or brokers advertising inventory for many sites.

Ad space
The location on a Web page reserved to display advertising. Multiple ad spaces can exist on a single page.

Adware
Advertising-supported software typically downloaded with other applications such as peer-to-peer file sharing.

Affiliate (also Publisher)
Individual or company that drives traffic to Advertiser/Merchant websites in exchange for a commission for each transaction generated: usually lead or sale.
 
Affiliate marketing
Marketing channel through which affiliates (publishers) earn revenue by featuring advertiser (merchant) offers on their websites or other media; publishers receive a commission for each advertiser transaction, e.g., a lead or sale, that originated from the publisher's website.

Alt (alternative) tag
Text that appears when a banner or another image doesn’t load or when the mouse cursor moves over a banner/image.