According to the specifications available from the IEEE (at http://standards.ieee.org/getieee802/), both 802.11a and 802.11b were ratified on September 16, 1999. Early on, 802.11a was widely touted as the "802.11b killer," as it not only provides significantly faster data rates (up to 54 Mbps raw, or about 27 Mbps actual data), but also operates in a completely different spectrumthe 5 GHz UNII band. It uses an encoding technique called Orthogonal Frequency Division Multiplexing (OFDM).
While the promises of higher speeds and freedom from interference with 2.4 GHz devices made 802.11a sound promising, it came to market much later than 802.11b. It also suffers from range problems: at the same power and gain, signals at 5 GHz appear to travel only half as far as signals at 2.4 GHz, presenting a real technical hurdle for designers and implementers. The rapid adoption of 802.11b and then 802.11g only made matters worse, since users of 802.11b gear didn't have a clear upgrade path to 802.11a (as the two are not compatible). As a result, 802.11a still isn't nearly as ubiquitous or inexpensive as 802.11g, although client cards and dual-band access points (which essentially incorporate two radios, or a single radio with a dual-band chipset) have come down in price.
The Wi-Fi alliance (http://www.weca.net) tried to call 802.11a "Wi-Fi5," but the name never stuck. These devices are also sometimes confusingly labeled "Wi-Fi," just like the completely incompatible 802.11b/g. Be sure to look for the specification's real name (802.11a) when purchasing gear.
802.11a can be significantly faster than 802.11b, but achieves roughly the same throughput as 802.11g (27 Mbps for 802.11a, compared to 20-25 Mbps for 802.11g). 802.11a would be ideal for creating point-to-point links, if devices with external antenna connectors were more readily available. Many people tout OFDM's ability to cope with reflections caused by obstacles (called multipath) as a good reason to use 802.11a, but 802.11g uses the same encoding while achieving greater range at the same power and gain. Some consider the shorter range of 802.11a to be a security advantage, but this can lead to a false sense of security.
Keep in mind that the 54 Mbps data rate is the theoretical maximum, and frequently is only achieved when in very close proximity to the AP. The speed scales back sharply as your distance from the AP increases, and suffers dramatically when separated by a wall or other solid obstacle. It is a very good idea to perform a site survey complete with throughput testing to determine whether 802.11a is suitable for your intended location.
It is probably a bad idea to build an 802.11a-only network unless you are already committed to using only 802.11a gear. If you want to allow guests to use your network, it is a very good idea to at least incorporate a few dual-band APs (or perhaps a dedicated 802.11g AP), as guest users are more likely to bring 802.11b or 802.11g gear with them.
Bluetooth, Mobile Phones, and GPS
Network Discovery and Monitoring
Wireless Network Design
Appendix A. Wireless Standards
Appendix B. Wireless Hardware Guide