In my lifetime, the humble shotgun shell has undergone a revolution in design. 

 

When I first started shooting as a young teen, you had shells made of cardboard with brass reinforcements at the base to hold the primers.  These shells were usually the "high brass" variety.  Wads were made of thick fiber and not plastic as it is today. 

 

The other kids of shells -- all brass -- were pretty rare and were reserved for military use.  The all brass shell came about during World War 1.  Cardboard shells got wet and jammed trench guns, so the military went to all brass.  The all brass shells were loaded with 00 buckshot and saw use through the Vietnam War.  They were gradually replaced by the current plastic and brass shells that are the most commonly found today.

 

Plastic shells started coming in during the late 1960's and have replaced just about every previous kind of shell.  I have seen some shells that were pretty much all plastic (without even low brass) that had a metal insert for the primer and that was it.  They were one-shot shells, much like the aluminum case CCI Blazer pistol cases.

 

If you get into shooting a shotgun with any regularity, you'll be a reloader.  Buying the amount of shells you'll typically go through shooting rounds of claybirds soon gets very, very expensive.  The average shotgun shell can be reused many times before the plastic becomes too brittle or splits.  The powder, primers, wads, and shot cost pennies and can be bought in bulk.  Some clubs sell reloads to members at deeply discounted prices.  If you're a shootgunner, rolling your own is the way to go.