steelhead are just the normal state of rainbow trout meaning they are andromonous. This means that, if given the chance, most rainbow trout will migrate to a larger body of water, normally the ocean. But, in the great lakes region they migrate to the Great lakes and once sexually mature will typically return to the same stream they were spawned in. However, some fish never leave their home streams and these are then called rainbow trout.
Typically they return to spawn for the first time at 3-5 years of age. And unlike many salmon species they do not die after spawning and can potentially return for quite a few seasons after that.