Interpret your server logs

Many hosters (web space companies) give you access to a software called Webalizer, with which you can get a broad overview of the number of visitors to your site.

If you need more detail to really understand where your visitors come from, what they were looking for (which keywords they typed into Google) and where they went on your site, then The FastStats Analyzer by Mach5 is your friend.

Faststats is one of the most advanced web log analysing software packages available. Use "wolfgangmittelmaier.com" as a discount code to get it at a substantially lower price.