The feature can be accessed at the iTunes Charts page or clicking on a link from the music store.

When you go to a preview page, you can see information for the album like the track listing, the length, and the price. There are also reviews that you can read through. If you go to the Charts page, you can search for other artists and songs and get to preview pages for those results. However, if you click on the link on the search results page that says view more results in iTunes, it will take you to the iTunes download page assuming you dont have it installed.