1. Overlays
2. Decals

Decals pretty much carry out the same function as overlays, except that they are not as versatile. The only reason they exist is for backward compatibility to older editions of Hammer (Worldcraft) and Half-Life 1 mapping. Their application is similar to overlays, with some minor differences.

As previously mentioned, overlays conform to deformation caused by displacement surfaces. In fact, decals cannot be applied to anything other than world solid brushes. In addition, decals take on the properties of the underlying brush. Decals (not overlays) pick up their alignment from the base texture of the face they are applied to. So if you rotate, flip, or scale the underlying texture, the decal gets transformed too. Frequently, they need to be re-applied if the base texture is altered.

Decal textures can be used as overalys, and vice versa. But decals are thoroughly obsolete. There really isn't any reason to use them anymore. So let's get on with the time and only use Overlays in our Source maps.

Finally, we'll be getting to the good stuff in our next tutorial, which will teach you how to make buyzones, bombsites, and hostages. Following that tutorial, you should be able to make a fully functional Counter-Strike map.