Further to the info provided by Andy, have a look at
this post on the official forums for more in-depth information on the various paint textures, and how they work.
In brief:
Main livery file provides the shadows ("ambient occlusion") on the car, plus any specific design elements. Also controls blending with the detail texture.
Detail texture is a small tileable texture that controls main body colour (usually only for single-colour cars, like the road cars) and specular spread of the material. Recommended to use the default texture for most cars (just don't include one in your folder) when making a livery, unless the car you're painting by default uses a coloured one (several Ferraris do this, annoyingly).
_Map texture controls reflectivity amount, reflectivity sharpness and specular intensity.
Different things on most race cars have different values of the above things, so some skinners will replicate that by manipulating these textures effectively. When trying to understand how to create a _map texture, remember that there's no real difference between an alpha channel and the RGB channels. In essence, a normal picture has three channels; red, green and blue. Each channel is a greyscale image where black=no light, and white=full light. So the red channel, for instance, white means 100% red light, black means 0% red light. Across the three channels the light from each colour is mixed to create the final full-colour image. Full black in the blue channel, but full white in the red and green channels, your full colour result would be yellow (because red light plus green light equals yellow; RGB are the primary colours, contrary to what your art teacher told you). White in all three channels means you get white light, because red plus blue plus green equals white.
As an extension to all of this, the skin system in AC is quite cool. When you make a new skin folder, you make a new skin. When you place a properly named texture in this folder, the game will use that texture instead of the car's default texture
for this skin only. With that in mind, every single texture on a car can be changed
per skin if you want to. If you don't want to, you just don't include that file in your folder and the game loads the default texture.
I hope that rambling made sense.
