The Munchkin Dice Bag is a textured nylon/vinyl/plastic dice bag. It's triangular (about 6.5 inches on a side) and is about 2.5 inches thick. On the middle of the right side is a long zipper that opens the bag. It comes in two flavors, tan with brown text and pink with purple text.
The bag has a special rule for use in Munchkin games: "When you take a Munchkin d6 out of the Munchkin Dice Bag and roll it, the Munchkin head counts as a 7, not a 1." This is referring to the special Munchkin d6s that come with Munchkin core sets, and which have a Munchkin Man head in place of the single pip for the "1". (These dice can also be purchased separately.) So while you can use the Munchkin Dice Bag to store a large number of dice, the special rule only applies to the d6s with the Munchkin head.
In addition to the special rule, the tan dice bag comes with two Munchkin cards. See the board game version for details.