Here is a quick overview over all the texture maps and their names that CC0 Textures uses in PBR materials.
Most PBR materials will have either all or some of the following maps:
The color map of the material. Some programs also call it "Albedo", "Diffuse", or "Base Color".
It defines the height/displacement of the material. Using this in combination with very-high-poly geometry allows for more detailed surfaces.
The normal map defines the normal vector of the surface at a given point.
Please note that the normal maps on CC0 Textures are in the DirectX-format. This may require manual changes depending on your software.
Ambient occlusion serves to fake the tiny, soft shadows that the material is throwing onto itself. This causes crevices to be a little bit darker. The ambient occlusion map simply gets multiplied with the color map. Therefore the dark parts of the map also darken the final color map while the white parts leave it untouched.
This map describes the roughness of the material. If this image is closer to black that means that the material appears to be very shiny. Consequently an almost white image means that the material has a very diffuse look.
A black and white map to define the opacity of the material.
Please note that some older textures use the _mask or _Opacity suffix for other purposes. In that case you can ignore them.
Defines whether a part of the surface is metallic or dielectric.
Less Common Maps
There are a other types of maps which are less common but still get used occasionally, for example on materials which emit light or on surface imperfections.
This map defines the light that a material emits.
Variants (_var1 / _var2)
Some surface imperfection materials use these suffixes to indicate different versions of the map, these are often inverted versions or versions with increased contrast.
Special Terrain Maps
Terrains have their own map system. The main Height Maps and Models do not have a map name beyond their resolution, but the Additional Maps do have names.
Models usually come with Color, Normal and Ambient Occlusion maps.