There are two kinds of click fraud audits, one for advertisers and one for publishers or networks.
The advertiser audit is generally a technical and human review of data collected from paid click visits. It is a one time event aimed at measuring the potential issue. For more details on this topic see click fraud detection.
The PPC publisher or network audit has different purposes. The goal is to attest the click quality and the fraud detection process to prove that clicks billed to advertisers are valid. It’s a matter of trust.
Publisher or network audits may be done by a click fraud specialist or by classic audit firms.
An example of a seal offered to audited publishers by a click fraud specialist: