A supermassive black hole is a perfectly ordinary black hole that happens to be really, really big.
How big? Well, The International Authority of Physical Cosmologists decreed… no wait, there is no such authority. I am not aware of any formal definition (Wikipedia says that a black hole is supermassive if it’s more than 100,000 solar masses, but that’s a pretty arbitrary choice and one not necessarily accepted by all cosmologists), but I think we pretty much know it is a supermassive black hole when we encounter it. Supermassive black holes, weighing typically several million suns and above, tend to lurk in the central regions of galaxies and who knows, may even have played a role in these galaxies’ formation. Stellar black holes, which usually weigh a few solar masses, probably mostly formed through core collapse supernovae, or larger ones, through the merger of existing black holes and/or neutron stars.