The CTR (Сlick Through Ratio) is a ratio of the clicks number to the number of shows being measured in a percentage. The CTR value is used for effectiveness of Internet advertising evaluation (banner advertisements, context advertisements et al.). The higher is this value, the more effective is the advertising.
For example, CTR=2 % means every 100 shows of a banner account for 2 transfers to a website being advertised by it. Usual banner is considered CTR in between 2-4 %.