Advertising online is great for quite a lot of reasons, but one of the best is that almost everything is measurable.

Unlike in print advertising, where you'll often have no idea of your ROI for a series of advertisements, online you can see who responded, how - and even more importantly, who didn't respond.  You can use the metrics and data to build a picture of how people flow through your site, and use this picture to understand any strengths and weaknesses that there might be.

It's important to think of your website like a sales funnel.  People enter the site, search for information, and ideally - contact you or buy a product.  We refer to the final step as a goal or conversion.  You want to make your sales funnel as simple as possible, because the fewer obstructions in the funnel (hopefully) the more conversions you'll get.  But how can you tell how people are using your site - what tools do you need?

There's a simple answer - you need Google Analytics.  This is a great package that can give you a comprehensive run-down on how visitors use your site - and best of all, it's free.