Combo is a trick that you can do with two to four cards. Then you can do some pretty good stuff to your opponent.

Too many times we get combos that require
a large ammount of cards to pull it off. While technically you can
still consider these as combos, seeing as how dificult it is to do the
combo, most of us don't even concider them real combos.

Combos, like:
Jowgen + Last Turn
Guardian Tryce + Soul Exchange
Creature Swap + any weak monster

These
are true combos. The harder it is to pull a combo off the less people will like your combo.