When a banner from another site is displayed on your site, your banner gets credits, and those credits allow your banner to be displayed on other people's sites. So people who are interested click on the banner and are sent to your site. A great banner exchange service is PowerExchange . It's FREE and has a lot of features other similar services don't have. Click the banner below for more info.
People also find sites by using search engines.
So if you want more visitors you should also submit your site to search engines. But it's boring to visit each search engine and submit your site manually.