a normal skintone is one shipped by Maxis or one that uses the Maxis default nude meshes; "shower-proof" skintones have been modified by a very clever hacking (discovered by the lovely Marvine - all praise her name eternally) that use custom meshes.
Most will have a custom icon that informs you they are linked in some manner to something. Below are examples of the icons you can expect to see.
When you are building a Sim in Create-A-Family or Create-A-Student, you click the * by the darkest Maxis skintone, select the one you want -- keep in mind you will not suddenly see the mesh it uses in a true nude state, that will only occur in game -- and that bit of doing is done.
If you find they are just a floating head in the shower, you have neglected to download the meshes or placed them in a location that the game cannot find them. For this reason nude state meshes should always be in the Downloads folder, not a folder inside the Downloads folder. Some versions of the game may or may not work the other way; but since I am not psychic I only give the advice I know to work 100% of the time.
So to recap - download the skintone you like, get the meshes, install all in your Downloads folder, start the game and get as twisted as you want to be making literally dozens of different versions of the same Sim.
PS: "shower-proof" is a common terminology, I suggest you take a moment to look at the genetics section of this sub-forum for further information
