The simplest test you can run will just load a singe page, and all its dependencies (images etc) over and over again. However, if you register an account you can create your own load scripts that load different pages, with pauses in between. Paying users also get access to the recorder feature which allows you to record a session that you run in a new browser window - you surf around your site and click on things, POST forms, etc and when you're done you have a load script automatically generated that will let your simulated clients do the same thing.
We don't currently have any "crawler" mode though, where the system tries to follow links etc on the site automatically.