There are some differences in material, and they do perform differently.
Natural (seal) skins are traditionally the best, providing traction without clogging with snow which makes them slip.
There are some plastic ones, like snakeskins(TM), that get mixed reviews. The snakeskins I used did not attach with glue, but strapped onto the ski and I did not like the function of them at all.
I'm sure there are more synthetic skins out there these days, but do not know the materials they use for them. They do make super-wide "skin-Stock" these days that must be trimmed to fit wider or shaped skis, it is important for the skins to cover the base, but not the edges.
There are also some sprays and cleaners that help the skins stay free of snow.
[This message has been edited by Roto (edited July 29, 2001).]</FONT>