Pay per Click (PPC) refers directly to those advertisements that accompany Search Engine Result Pages (SERPs) generally along the right hand side as well at the top of the organic results. In Google the ones just above the organic results are currently shaded in yellow.
There are really only two metrics that matter when it comes to PPC … (1) CTR (Click through Rate) and (2) Conversion Rate. Click through Rate is simply the number of times a paid ad is clicked divided into the number of times the paid ad is displayed (impressions). Conversion Rate is the number of conversions (leads, sales, filled forms, subscriptions, etc) divided into the number of paid visits, generally to a specific landing page. In essence, PPC success can be fully assessed by focusing just on these 2 metrics.