1) an ad which is served to a user’s browser. Ads can be requested by the user’s browser
(referred to as pulled ads) or they can be pushed, such as e-mailed ads; 2) a measurement of responses from an ad delivery system to an ad request from the user's browser, which is filtered from robotic activity and is recorded at a point as late as possible in the process of delivery of the creative material to the user's browser -- therefore closest to the actual opportunity to see by the user. Two SEO methods are used to deliver ad content to the user - a) server-initiated and b) clientinitiated. Server-initiated ad counting uses the publisher's Web content server for making requests, formatting and re-directing content. Client-initiated ad counting relies on the user's browser to perform these activities. For organizations that use a server-initiated ad counting method, counting should occur subsequent to the ad response at either the publisher's ad server or the Web content server. For organizations using a client-initiated ad counting method, counting should occur at the publisher's ad server or third-party ad server, subsequent to the ad
request, or later, in the process. Click here for the IAB's Ad Campaign Measurement Guidelines.