1. Measure bounce rate
As Avinash described this is essentially when someone comes to your website, vomits and leaves. OR in a less disgusting but not as funny way this is a visitor who having not found what they were looking for left the site immediately.
Be sure to measure bounce rates to understand the number of individuals who are abandoning the website. While some of the traffic will be irrelevant and therefore always likely to bounce, the majority are bouncing because the information they want (while it may be on your website) is not at their fingertips.