Report Builder is a client-side application that you can use to create and design ad hoc reports. Using Report Builder, you can select data and design reports without the use of any any complex programming languages. You simply need to be familiar with the business model of your data.

See these links for information on Reporting Services and ad hoc reporting:

Ad Hoc Reporting with Report Models
Ad-hoc Reporting (video)
2005 Reporting Services page
Reporting Services Tutorials
Reporting Services Books Online
Bob Meyers Reporting Services Blog