Online advertising is the paid distribution of messages across digital channels to drive measurable outcomes such as awareness, leads, or sales. At the most basic level, it is built on a few primitives. An impression is a single ad view opportunity served, or counted as served, to a user or device. A click is a user interaction that sends them from the ad to a destination such as a website, app, or store. From these two primitives, derived ratios become the foundation of performance analysis. Click-through rate (CTR) is the ratio of clicks to impressions, while conversion rate (CVR) is the ratio of conversions to clicks, though the exact denominator should be defined consistently across reports.
A conversion is any tracked action aligned to a business goal, such as a purchase, lead, signup, or install. Because a user may see or click an ad long before converting, advertisers define a conversion window: the period after an ad interaction during which a conversion can still be attributed to that interaction, for example a 7-day click window or a 1-day view window. The customer journey is often modeled as a funnel with three stages, top of funnel (TOFU) for awareness, middle (MOFU) for consideration, and bottom (BOFU) for conversion, and each stage calls for different creative, targeting, and measurement approaches.
When a user clicks an ad, they typically arrive on a landing page designed specifically to convert that traffic. The most important principle of landing page design is message match: the promise and creative of the ad should be consistent with what the user sees on the page. Strong message match reduces bounce rates and improves conversion rate, while mismatched ads and pages erode the value of the click.