802.11, also known as IEEE 802.11, is a set of standards to allow different devices to communicate wirelessly. It is created by the IEEE (pronounced eye-triple-e) LAN/MAN Standards Committee.
All 802.11 signals are centered around the 2.4, 3.6 and 5 GHz frequency bands.
There are multiple "flavors" of 802.11, and even though they have different letters as suffixes, it doesn't mean that one necessarily supersedes another.
802.11a and 802.11b were released in 1999, and are still widely popular. 802.11a is centered around 5 GHz and has a range of roughly 115 feet indoors. It has a high data rate potential, upwards of 54 Mbit/s is possible.
802.11b is the most common of the different 802.11 versions, and is the de facto standard. Nearly all devices that have some 802.11 standard will be able to b as well. 802.11b is centered on the 2.4 GHz range, and has a maximum data rate of 11 Mbit/s. One potential problem is the 2.4 GHz band is fairly crowded, with microwaves, Bluetooth, cordless telephones, and baby monitors all occupying this range. Most modern devices will "frequency hop" to avoid conflict with other devices. The maximum potential range of 802.11b devices indoors is around 125 feet.
802.11g os also centered around the 2.4 GHz band, but uses a transmission scheme that's the same as 802.11a. As such, it takes pros and cons from both. It has the same potential range indoors as b, but the maximum data rate potential as a.
802.11n is the newest standard, adding multiple antennas that function in a "MIMO" mode, or multiple-input multiple-output. 802.11n works on both the 2.4 GHz and 5 GHz bands. Extremely high data rates are possible, potentially upwards of 150 Mbit/s. Range is also increased, with a possible range of 230 feet indoors.
Devices with wireless capability will have a label such as "802.11a/b/g" which means they are able to communicate with the a, b and g standards. This only matters so long as your wireless router shares at least one of the same letters. If bandwidth is an issue (transmitting HD wirelessly, for example), a specific letter may be needed and will be listed on the product.
For even more information, check out the Wikipedia page.