A decoy is useful for making a mob farm for example, because mobs go to them and try to kill them like if they where players! or you can put them around your base or near to a village because mobs will attack them first. And they're really easy to make! in your first 10 minutes you can make some Iron decoys or hail, which are cheaper. When a decoy dies, there is a 100/100 chance to drop back the decoy as a item, so it isn't a big problem when a mob kills a decoy, just pick up the item before it despawns.

There're two types of decoys, those of hay and those of iron

To make the hail one you will need to do this:

and the iron one you will need this:

A important thing to say is that the hay decoy only atract zombies and have 10 full hearts of live, but the iron decoy atracts all type of mobs and have 25 full hearts.