Matrix Reports (Matrix By Groups)

A matrix (cross tab) report contains one row of labels, one column of labels, and information in a grid format that is related to the row and column labels. A distinguishing feature of matrix reports is that the number of columns is not known until the data is fetched from the database.
To create a matrix report, you need at least four groups: one group must be a cross-product group, two of the groups must be within the cross-product group to furnish the "labels," and at least one group must provide the information to fill the cells. The groups can belong to a single query or to multiple queries.

A matrix with group report is a group above report with a separate matrix for each value of the master group.

A nested matrix (crosstab) report is a matrix report in which at least one parent/child relationship appears within the matrix grid.
The new Child Dimension property of the nested group enables you to eliminate empty rows and/or columns in your single-query nested matrix.

Types of Matrix Reports
Simple Matrix Report: Is a matrix with only two dimensions
Nested Matrix Report: Has multiple dimensions going across and/or down the page
Multi-Query Matrix with Break: Is similar to a nested matrix report in that it has more than two dimensions. Does not display records that do not contain data
Matrix Break Reports: Contains a new matrix for each master record