Wireless networking is an easy way to connect houses and offices upto several kilometers apart.
You can create broadband and file sharing networks between, in and around buildings without the need for expensive and unsightly wiring.

Range
You will see specifications for different brands quoting wildly different ranges, take these claims with a pinch of salt, unless they have something very wrong or are operating at illegal output levels products from different sources will all behave roughly the same

radio waves travel in straight lines and do not penetrate obstacles very well, some surfaces reflect some absorb.
Water and organic matter that contains water like trees and humans are very good at absorbing signal.

metal reflect the signal, modern houses may also have foil backed plasterboard to meet fire regulations and modern glass has a metal content.

This said, as a general rule 802.11b/g WiFi devices will cover a house quite well, not guaranteed though.
Bear in mind if your antenna is stuck down behind the computer case if already has two layers of steel or a wall to penertrate first.

When you replace the antenna onto a wireless device, the antenna gain acts on both the transmit power and reciever sensitivity, so you are not only sending your signal further but you are also able to recieve much weaker signals.

Line of sight range in free space (outside)
gain       range
0dB      200m
4dB      440m
7dB      620m
10dB    1.2km
13dB    2.8km
16dB    5.0km

A Directional antenna produces gain by focusing the radio energy in one direction.
Image the output to be like the beam from a torch.
It start off from a point and the then slowly the beam get wider and weaker as it travels further, the higher the gain the stronger the signal, like a big high powered torch. the direction you point it in is the only wireless network area you will see.
Very good for creating building to building links without sending your network signal all over the place.

An Omni-direction antenna radiates a 360 degree disc shape signal by stealing power from above and below.
Image this output to be like a ripple on a pond with a dimple in the top and bottom.
these are used more for access points in a central location. you would generally set one omni antenna in a central location in the network and have direction antenna pointing in toward this antenna to create the widest possible network spread.