They provide you with an HTML codelet that you insert into your pages. Now when someone visits your website, they display someone elses banner on your pages. When enough people visit your site, it triggers the ABE to display your banner on someone elses website. Not really, lets look at some basic facts, then do a little math. So for every 100 flyers, figure you get 1-2 customers.
Banners work in a similar manner.
For every 100 banners of yours that are displayed, you get 1-2 people to click on them. This means that for every 2 banners you display, one of your banners gets displayed. Purple Monsters markets a brand of childrens clothing for unruly kids. Their website has been online for over a year and they are doing nearly 1000 visitors per month, or roughly 250 visitors every week. Fairly average for a online small business.