Test Matrix can be defined as large pool of Test Cases with various possible combinations (each case having it's own Unique ID)depending upon the all the avaliable configurations.
The depth and utility of each Test Case depends upon to what extent (the Design specfication of a particular system) the System is involved.
For example if you are testing an Web Application which receives an HTTP request through a browser. So if Operating systems( Win 98, 95 etc) are considered on one side and if the Browsers(NS, IE, AOL etc.) are taken on the other side, the cross-section of each combination forms a Unique Test case. So this entire combinations of Test cases can be defined as an Test Matrix.