Starting with 7 cards, all you need to do is discard them one at a time by matching the color or number of the last card played.  Sound easy?  Well, don't forget the Skip, Reverse and Draw Two cards.  Not to mention your opponent can play a Wild or Wild Draw Four at any time and change the color on you.  And don't forget to say "Uno" before you play your next to the last card or you'll be drawing more.  Needless to say, your fortunes can change instantly in this game.