Like most people you're probably using the standard antennas that came with your WAP. These antennas provide you with a few hundred feet of signal transmission strength, as long as it is not impeded by walls or other type of interference (such as microwaves, other Wi-Fi networks, etc..). By using a better antenna, the farther your signal will go and the better it will be received.
The needs of your Wi-Fi network, will determine the type of antenna you need. For example, there are several different types of antennas, like: unidirectional, omnidirectional, etc.. Each type of antenna, does something different, the omnidirectional antenna send the signal out in all directions, and unidirectional only sends the signal in one direction.
Wi-Fi Antenna Resources:
- FreeAntennas.com - DYI antenna resources
- Metrix Communication LLC - 802.11 wireless networking gear
- Building a Wi-Fi Antenna Out of a Tin Can
- Wi-Fi Antenna made from a Diskette