Overlays are usually graphics that are larger than the typical 2500 pixels (or 5000 for CS6+) for brushes. In some cases, they are in color.
Brushes are compatible with CS2 and above and PS Elements 6 and above. I occasionally have CS6 only brushes — they will be indicated.
Most of the brushes come with PNGs for unsupported software, but I'm phasing this out. For PNG extraction of the brushes, I recommend the free app ABRMate for PC users or the modestly priced Brush Pilot for Mac users.
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.