characteristics of yagi antennas

Yagi uda antenna is having high directive gain.

It have one dipole and many reflectors.

http://www.qsl.net/dk7zb/

This guy has all the info you will need about Yagis (mostly VHF but you will get the idea)

http://en.wikipedia.org/wiki/Yagi_antenna

In short:
the boom lenght determines the maximum gain of the antenna. By adding more passive elements you can alter the lobe pattern, optimizing for certain goals i.e. lowest side-lobe.