Unlike the triangular or rectangular teeth you see on comical depictions of gear trains, gear teeth are actually ground to a shape called an "involute".
If you have a string wound around a cylinder, and you keep that string tight as you unwind it from the cylinder, the shape of the curve the end of that string makes is called an "involute". It's a shape similar to a spiral.
The unique advantage of involute gear teeth is that when two involute gear teeth meet, they actually ROLL over each other. That is, the point of contact between the two teeth is a ROLLING contact, not a sliding contact.
And it's this rolling contact made possible by the involute shape of the gear teeth that results in negligible loss of mechanical power to friction, and therefore the excellent mechanical efficiency of gear trains.
If gear teeth slid across each others surface, as is depicted in comical gear trains with rectangular teeth or the "peg" shaped gear teeth you might see on a TV show like "Gilligan's Island", then much of the mechanical power would be lost to friction.
Watch the gear animation at the top of this web page: 'Involute gear - Wikipedia, the free encyclopedia' (http://en.wikipedia.org/wiki/Involute_gear)
The black dot is the point of contact between any two gear teeth, and it's the involute shape of the gear teeth allows them to roll over each other, thereby transmitting power with negligible friction losses.