Four stages comprise a Click-Referral Cycle:

1. Initiated Click – When a user is served and interacts with an Advertisement or search result, including impression content or other descriptive information. In concept, prior to an initiated click, the user is served either (1) an Advertisement consisting of an Ad Impression, or (2) search results consisting of descriptive information (most frequently an HTTP GET to Publisher’s or Publisher’s Agent’s server) following a Search Transaction.

2. Measured Click – Following an Initiated Click additional information is sent to an Advertiser or Advertiser Agent’s site or to the user on the Publisher’s site. At this stage, the Publisher or Publisher’s Agent may count the Click for reporting purposes (for example, a successfully sent HTTP 302 re-direct from a Publisher or Publisher’s Agent or a received HTTP GET from a user’s browser as a result of an initiated click), subject to the click filtration controls described below.

3. Received Click – When a Measured Click reaches an Advertiser’s site and is measured by an Advertiser or an Advertiser’s Agent (most frequently an HTTP GET).

Note: Within certain search or shopping transactions a Publisher may acquire Advertiser content and maintain this content for presentation to the user, which is essentially a form of content caching. If clicks are initially directed to this content hosted on the Publisher site rather than the Advertiser’s content directly, no Received Click occurs. The Publisher uses other means, and in some cases alternative measurement metrics, to communicate this activity to the advertiser for payment. Additionally, in these situations Resolved Clicks as described below may not occur, until the user’s actions execute further redirects.

4. Resolved Click – When a Received Click launches an Advertiser destination web page or other web page intended to provide the Advertiser with a commercial interaction with the user (for example, a lead referral form hosted by the publisher on behalf of the advertiser). The final destination may also be an FTP or HTTP download (most frequently an HTTP 200).

Source: IAB Click Measurement Guidelines