Typically a detail map is a smaller texture (say 256x256) that tiles hard across the surface of your model with a high texel density. They are used in next gen-shaders togheter with a bigger size diffuse or color map for variation. u blend the two textures togheter in the shader (say multiply the diffuse on the detail)…
Detail maps are usually a high-frequency tiling pattern which is overlaid onto a main texture. Gears of War used this a lot for adding close-up detail when players were up against walls. It can help disguise low-res textures. In 3dsmax you could probably set them up using a "blend" or "mix" material, with a tiling version…