A Mylar D bag is actually a brand name.  It is a
form of "plastic" that was originally used for
things like Magazines, bookes, etc.  They are acid
free, tend to be much stiffer than the usual
"Ploly-whatever-they-are-called" bags that you
usually find.  Mylar D cels tend to creat a lot of
static due to the stiffness of the plastic bags.

What I found to be the best was that
"Ploly-whatever-it's-called" bag (sorry, don't
know the name, almost called it a polypeptide! 
LOL!) which are usually what the studios use to
store them in.  I have never had a problem with
them except for whenever the adhesive gets stuck
on the cels whenever I try to remove them.  But,
it is easy to work with and they are cheaper,
softer, and the cels look nice in them.

Star Phoenix