This guide is designed to stand on its own as an educational reference. It separates technical concepts, commercial outcomes and policy considerations so readers can reason about the system rather than memorize terminology.
Start with the denominator
Advertising metrics are ratios. The numerator tells you what happened; the denominator tells you what it happened against. Confusion usually starts when teams compare numbers with different denominators. Before interpreting a metric, write out the formula. That simple habit prevents many reporting mistakes.
CPM
CPM measures spend per 1,000 impressions. If an advertiser spends $500 for 100,000 impressions, the CPM is $5. CPM is useful for comparing the cost of reach and inventory, but says nothing by itself about whether those impressions were viewable, relevant or effective.
CPC and CTR
CPC is spend divided by clicks. CTR is clicks divided by impressions. A campaign can have a low CPC because clicks are plentiful, but that is not automatically good if clicks are accidental or low quality. A high CTR can also be undesirable if the creative misleads people into clicking.
CPA and conversion rate
CPA is spend divided by conversions. Conversion rate is usually conversions divided by clicks or sessions, depending on the design. The denominator should always be stated. A conversion also needs a precise definition: purchase, completed application, verified sign-up and page view are not equivalent outcomes.
Use a metric stack
A practical scorecard can include delivery volume, CPM, viewability, CTR, CPC, conversion rate, CPA and post-conversion quality. For video, add completion and quartile metrics. For rewarded experiences, add eligibility and fulfilment outcomes. Metrics should form a chain from opportunity to outcome.
Beware blended averages
Averages can hide important differences between countries, devices, placements and sources. A stable blended CTR can hide a severe drop in one market. Segment before concluding. The most useful metric is often the variation that explains why the average moved.