802.11 is a standards group under the IEEE that develops standards related to wireless and wired Ethernet transmission. This includes the actual physical layer, including 802.11a and 802.11b modulation schemes.
802.11b is a Direct Sequence Spread Spectrum technology that, in the United States, occupies 11 channels centered on frequencies in the Industrial, Scientific, and Medical (ISM) band from 2.412 to 2.462GHz, in 5MHz steps. The spectrum used by a single 802.11b signal is 22MHz wide. Because the channels are smaller than the occupied bandwidth, only three channels (1, 6, and 11) can be used in a small area without running into interference.
802.11a doesn't use Direct Sequence. Instead it uses a modulation scheme called Orthogonal Frequency Division Multiplexing (OFDM). OFDM uses 52 300KHz-wide carriers grouped into one 20MHz-wide channel. The slower symbol speed of OFDM and the forward error correction incorporated into 802.11a make it more resilient to multi-path and interference. However, because 802.11a is broadcast at more than double the frequency of 802.11b, there is greater free space loss. An 802.11a installation with gain antennas and powered transmitters has a signal strength that is about 18 percent weaker than that of a similar 802.11b setup.
While 802.11b occupies the portion of the ISM band at 2.4GHz, 802.11a can occupy either the ISM band at 5.8GHz (5.725 to 5.850GHz), or a section of spectrum known as the Unlicensed National Information Infrastructure (U-NII) band. This band was approved in 1997 and promoted by the group WINForum, which was made up of individuals and companies such as Apple Computer.
The band takes up 300MHz of spectrum and is divided into three 100MHz sections. The first two are next to each other, and the third is 375MHz up from the top of the second band. The "low" band runs from 5.15GHz to 5.25GHz, the "middle" band runs from 5.25GHz to 5.35GHz, and the "high" band runs from 5.725 GHz to 5.825 GHz. 
 The FCC currently has a Notice of Proposed Rulemaking, or NPRM (RM-10371), to add 5.470 to 5.725GHz to the U-NII band.
 Note that the ISM band actually goes another 25MHz higher than the "high" portion of the U-NII band.