Overlays are usually graphics that are larger than the typical 2500 pixels (or 5000 for CS6+) for brushes. Brushes are compatible with CS2 and above and PS Elements 6 and above. (I occasionally have CS6 only brushes - they will be indicated.)
Learn how to easily install and use brushes in this video tutorial.
Learn how to easily use and change colors with transparent overlays in this video tutorial.