Uninsured Motorist (UM) coverage is there to pay for your injuries if you’re struck by a hit-and-run driver or, as is more often the case, by someone driving without car insurance. Most carriers require some kind of confirmation that the party at fault is uninsured before this coverage will kick in.
Underinsured Motorists (UIM) coverage will pay out if the driver at fault causes more damage than his liability coverage can cover. In some states, UM or UIM coverage will also pay for property damages.

UM coverage is required in a growing number of states. UIM coverage, however, is only required in four: Connecticut, Minnesota, Maine and Vermont.