Decals are special textures that are placed on brushes to help break the monotony of a repeating texture or just to add small details. When you shoot a wall in the game, decals are applied to the places where the bullets hit. Unlike normal textures, they don't have to occupy an entire brush face. When a decal is placed in the 3D view, a point entity infodecal is placed at the mouse cursor's location, and the decal texture is projected onto all surfaces within 16 units of the infodecal origin.

Decals have special render properties that separate them from normal textures. It's not a good practice to apply a regular texture as a decal because the scaling is different. When a decal texture is placed with the decal tool, the texture is scaled down. This scaling is specified when creating the decal texture. When standard textures are applied, there is no scaling taking place. There's nothing stopping you from applying a standard texture as a decal or even creating a brush with...