In Report Builder, a group is a named set of data from the report dataset that is bound to a data region. Basically, a group organizes a view of a report dataset. All groups in a data region specify different views of the same report dataset.
To help visualize what a group is, refer to the following figure that shows the tablix data region in Preview. In this figure, the row groups categorize the dataset by product type and the column groups categorize the dataset by geographic region and year.