Most OS's support WEP and WPA. WEP is either 40-bit or 104-bit. The extra 24 bits that are advertised are just extra bits tacked on the end of the key. You can crack WEP using basic/semi-advanced brute force, basically guessing. WEP is a RC4 algorithm, a stream cipher. A stream cipher uses a key, then encrypts the data using XOR, exclusive OR. So all you have to do is to gather the data, then taking a thousand to million keys, and XOR to figure out the data. This can be done using your laptop/desktop and about an hour or two.
WPA is more advanced. It still uses RC4 cipher, uses TKIP, Temporal Key Integrity Protocol, which changes the key as the system uses it. It still isnt unbreakable, but it is better, since it is less known, and has built-in security measures.
A little googling, and some Wikipeding will answer any techinical questions.
When I setup wireless networks, I use MAC filtering, and WEP/WPA. Nothing is gaunteed to keep the bad guys out. FWIW, wired LANS can be tapped, just cut off the insulation on the wire, and attach clips, even fiber can be tapped. If somebody is willing enough, they will succeed.
Now, MAC filtering is not a sure thing either. Anyone who is useing a wireless interface can be seen, you just have to know how to look.