From Wikipedia:

Clickthrough rate (CTR) is a way of measuring the success of an online advertising campaign. The clickthrough rate of an advertisement is defined as the number of clicks on an ad divided by the number of times the ad is shown (impressions), expressed as a percentage. For example, if a banner ad is delivered 100 times (100 impressions) and receives one click, then the clickthrough rate for the advertisement would be 1%.

Click-through rates for banner ads have fallen over time. When banner ads first started to appear, it was not uncommon to have rates above five percent. They have fallen since then, currently averaging closer to 0.2 or 0.3 percent. In most cases, a 2% click-through rate would be considered very successful, though the exact number is hotly debated and would vary depending on the situation. The average click-through rate of 3% in the 1990s declined to 0.28% by 2003. Since advertisers typically pay more for a high click-through rate, getting many click-throughs with few purchases is undesirable to advertisers. Similarly, by selecting an appropriate advertising site with high affinity (e.g. a movie magazine for a movie advertisement), the same banner can achieve a substantially higher CTR. Though personalized ads, unusual formats, and more obtrusive ads typically result in higher click-through rates than standard banner ads, overly intrusive ads are often avoided by viewers.