802.11a came after 802.11b (don't ask!), and operates at a much faster rate (up to 54M bits/second on a very good...
day) and higher frequency (5GHz). As a result, there is less of a possibility for interference from other wireless networks (including Bluetooth) and common wireless devices such as garage door openers and cordless telephones (which also operate in the same 2.4GHz spectrum). Not only can 802.11a pump information faster, but also relies on a different frequency multiplexing scheme than 802.11b. The downside of this is that 802.11a is not backward compatible with 802.11b, which means you essentially have two different wireless systems installed -- one for more generic and less-critical information (802.11b), and the other for more multimedia-rich and possibly more sensitive data (802.11a).
The newest entry in the wireless LAN arena is 802.11g, which operates in the 2.4GHz radio frequency band (like 802.11b), but is capable of speeds up to 54M bits/second -- again a theoretical goal, but not really attainable in the real world. 802.11g is actually an extension of the OSI Model Physical Layer of 802.11b. As a result, it is completely backward compatible, which means you can by 802.11g routers and access points today and plug them into existing 802.11b wireless networks. Products are already available from the leading network makers, and they are not much more expensive than 802.11b devices.
While there will definitely be a place for 802.11a within the enterprise, the smart money is on the fast rise of 802.11g as an obvious successor to 802.11 networks both in the enterprise and in the home. In fact, right now 802.11g products account for about 20 percent of all wireless LAN shipments in the U.S. 802.11g chips also feature better power management than its predecessors, which means it is a more viable choice for mobile devices with limited battery capacities. Oh yes, about that interference problem mentioned earlier: There is the potential for 802.11b and 802.11g devices to suffer some performance degradation because of the rising number of devices operating within the 2.4GHz spectrum. In fact, in some tests, Bluetooth -- which is a 2.4GHz short-range wireless technology designed to replace cables and wires -- knocks the wireless stuffing out of 802.11 by dominating the signal pathway. Not to a great extent, but enough to create a measurable blip in some cases. So, you may see hospitals and utilities opting for 802.11a and other faster, more spectrum-exclusive and perhaps more secure flavors now under development to avoid the possibility of signal collision and possible data corruption.