For a general, not too technical introduction, look at http://www.homenethelp.com/802.11b/index.asp

For a good discussion of the physical and MAC link layers look at http://www.sss-mag.com/pdf/802_11tut.pdf

For a tutorial given by the IEEE Working group on 802.11 itself, see http://grouper.ieee.org/groups/802/11/main.html

3COM has a website which gives an overview of 802.11 at http://www.pulsewan.com/data101/802_11_b_basics.htm

The physical layer is discussed on http://www.wi-fiplanet.com/tutorials/article.php/2107261.

A site discussing 802.11 from microsoft that gives a good overview: http://www.microsoft.com/resources/documentation/WindowsServ/2003/all/techref/en-us/Default.asp?url=/resources/documentation/windowsServ/2003/all/techref/en-us/w2k3tr_wir_how.asp

The following website has many other websites that discuss 802.11. http://www.cs.columbia.edu/~hgs/internet/wlan.html