Insimenator.org

Objects in 3D games are comprised of "geometry" (or "models") and textures - two dimensional pictures or images that are applied to the surface of that geometry to give it a particular look.  In 3D games, object geometry is actually made up of tiny triangles "glued" together to make a single contiguous surface.  This is traditionally called a "mesh" - a latticework of triangles that make up 3D geometry:

(http://mysite.verizon.net/aestudios/female_mesh1.jpg)

If we solidify this "mesh" we get our mannequin that Simmie spoke of:

(http://mysite.verizon.net/aestudios/female_mesh2.jpg)

We can then apply a texture (clothing, in this case) to our model to give it a more appealing look than dull gray :)

(http://mysite.verizon.net/aestudios/female_mesh4.jpg)

The 2D texture is stretched across the surface of the geometry, producing the end product:

(http://mysite.verizon.net/aestudios/female_mesh3.jpg)

As SimmeCal mentions you might download one mesh, and then multiple textures (or skins) which are applied to that mesh to create many different looks.

In short:

A mesh defines shape

A texture defines style

You need both to see anything in the game.  Many (if not most) custom clothing or content just uses the mesh files that are already in the game.  But if you want a differently shaped object - one not present in the game by default - then you need a custom mesh for that object as well.  Many custom body styles, hair, and clothing require the use of such meshes.

J