Dennis, I was all set to question your sanity for spending $65 on a soup can, but after looking at the ad, I see that some other parts are included to make this a complete wifi adaptor for your computer.
"Cantenna" is the common name for a waveguide antenna, which are about the easiest type of antenna for a home experimenter to construct. Sice wifi frequencies are 2.4 GHz, the size of the can and the active element inside are very conveniently small.
I've built several of these antennas, although I use 3" copper pipe with copper sheet silver brazed to close the one open end. Copper is much better for corrosion resistance, the antenna pictured above would be fine for indoor use, but would rust quickly if used outside.
Waveguide antennas can be made in ¼, ½, and full wave configurations. The photo below shows one of each. I've posted these photos here before, but here they are again. I use old satellite dishes that have been slightly modified to act as parabolic reflectors, boosting the total gain to somewhere around 14db (a 25 times increase in power). The first three antennas are shown on the front porch of my old house with a commercially manufactured mesh antenna for comparison. Performance of these antennas was identical to the commercial antenna.
The largest of my antennas uses a 10 GHz microwave reflector and a full-wave waveguide antenna.
If your "cantenna" helps, but isn't quite enough to pull in weak signals, consider putting a reflector on it. Even a small "dish network" type reflector will help a whole lot.
One note: if used outdoors, you ~must~ somehow cover the open end of the antenna. If you don't water will get inside and foul things up and insects (particularly wasps and spiders) will make it a home, degrading your signal. Just wrapping some saran wrap over the end and securing it with a rubber band is a good temporary solution, although the sun will degrade those materials quickly. "Tyvek" type house wrap and a stainless steel hose clamp are purported to be a permanent solution.
Our own Ezrablu turned me on to this great little device and it works great. The best 65 bucks I spent lately 
Yes folks, The "Can-Tenna" It wont slice or dice a tomato but you can watch someone else slice and dice one on the food network from your free wifi 
