Click-thru rates

I am currently doing a research paper on click-thru rates. I was
wondering if anyone could answer or provide a link to answer the
following questions.

1.A definition/explanation of click-thru rates
2.An explanation of why the topic is important to marketers
3. A determination of a realistic range of click-thru rate
expectations that could be used by marketers (i.e. give me a ballpark
figure or range of typical click-thru rates).

Thanks,

Brent