I have been asked about parameters in reports and although I am presenting a TechNet session An introduction to SQL Server 2005 Reporting Services  on the 6 Sept here in Reading  I thought it would be useful to show where some of the resources are for parameters.  Parameters are used to provide user interaction with your reports and this can be implemented in various ways.  There are two types of parameters in a report: query parameters and report parameters:

Query parameters are used to reduce the amount of data returned to the report from a data source

Report parameters allow you to select or enter values as a report is processed.

There is a link to working with parameters in Reporting Services  and also a link to provide user interaction in reports.  there is an excellent blog by Bob Meyers about reporting services you may be interested in.