Both radio-based versions of the 802.11 specification transmit in the unlicensed 2.4GHz band in the United States and Canada. The 2.4GHz band is reserved for industrial, scientific, and medical (ISM) purposes, meaning that there's a wide variety of devices besides wireless LANs in the band. The other two ISM bands are at 900MHz and 5.6GHz.
Until recently, the 2.4GHz band was relatively quiet, especially in comparison to the 900MHz band, which has a lot of traffic for wireless telephones. Some of the most recent systems using the 2.4GHz band include newer wireless telephones and the Bluetooth cable-replacement system targeted at computers, telephones, stereos, and other applications. All in all, the 2.4GHz band is becoming noisier, which means that some locations will be less reliable for wireless radio LANs running near maximum distance than others.