Skyrim's texture folder is littered with files that have underscores. I have a general idea what they do. Happy modding!
texture_n
Normal Map. This type of map is used to add details to an object without adding more polygons to the model. It saves space and creates a nice effect. (http://en.wikipedia.org/wiki/Normal_mapping)
texture_s
Specular Map. Defines the highlights and shininess. (http://wiki.splashdamage.com/index.php/Specular_Maps)
texture_d
Diffuse Map. Defines the surface's main color. (http://wiki.splashdamage.com/index.php/Diffuse_Maps)
texture_g
Many thanks to Werne for pointing these out. They are glow maps for textures that have a glow to them. (http://bighouse.bungie.org/killingtime/Help_Schtuff/Glow_Maps.html)
texture_p
Parallax Map. The main purpose of these files is to provide greater depth to things such as walls, creating more realism without impacting performance. (http://en.wikipedia.org/wiki/Parallax_mapping)
texture_e
Environment Cube Map. See the wiki for this one, it's a bit complicated. (http://en.wikipedia.org/wiki/Cube_mapping)
texture_b
Blurry? More research needed.
texture_msn
Model Space Normal map/Object Space Normal map.
texture_sk/s
Stands for "Skin". I'm not sure what purpose this file serves. Ghogiel seems to think that it's a sub-dermal layer, placed underneath the already slightly transparent textures.
texture_em/_m/_envmask/_envmapmask
Environment Mask Map.
If anyone has any further knowledge, I'll be happy to add it here. There aren't many topics about this subject, so let's make it happen people!
Credits:
Werne - For reminding me that _g maps actually exist.
Ghogiel - For great information on all maps listed.
Throttlekitty - For pointing out the _b textures.