Imagine a left hand turn. Oversteer kicks out the rear wheels to the right leaving the car pointed too far left of the target. Result: rear end spins out. Understeer allows the front wheels to drift to the right leaving the car pointed too far right of the target. Result: couldn't make the turn.
Understand that the results can be highly variable; from correctable in the turn with steering and/or throttle all the way to complete loss of control.
If you watch nascar, they call this loose (oversteer) and tight (understeer).
What it feels like......well, sometimes "Yeeehaahhh".....sometimes "Oh, crap!"
--Case...