Melanie a crate is another name for a cage yes. Some of them are made of plastic others of metal.
Crate training a dog is to keep them out of mischief and give them their own space of safety and security it also keeps them from getting hurt...especially when you aren't home to supervise them.
It's most ideal for dogs that have been recently adopted from shelters, puppies or dogs with normally destructive behaviours due to separation anxiety and such.
I hope that helps a little.