[http://www.networkmagazine.com/article/NMG20020206S0006/1 Network Magazine] had an excellent review of different 802.11x standards. Below is their explaination verbatium:
The various IEEE 802.11 standards can be confusing, to say the least. In addition to the three main specifications that define complete wireless LAN systems (802.11a, 802.11b, and now 802.11g), the IEEE is working on enhancements that mitigate weaknesses in the existing protocols. These aren't new wireless LAN systems, but rather extensions that will eventually be applied to one or all of the existing three.
802.11d aims to produce versions of 802.11b that work at other frequencies, making it suitable for parts of the world where the 2.4GHz band isn't available. Most countries have now released this band, thanks to an ITU recommendation and extensive lobbying by equipment manufacturers. The only holdout is Spain, which may follow soon.
802.11e will eventually add QoS capabilities to 802.11 networks. It replaces the Ethernet-like MAC layer with a coordinated Time Division Multiple Access (TDMA) scheme, and adds extra error-correction to important traffic. The technology is similar to Whitecap, a proprietary protocol developed by Sharewave and used in Cisco's 802.11a prototypes. A standard was supposed to be finalized by the end of 2001, but has run into delays thanks to arguments over how many classes of service should be provided and exactly how they should be implemented.
802.11f tries to improve the handover mechanism in 802.11 so that users can maintain a connection while roaming between two different switched segments (radio channels), or between access points attached to two different networks. This is vital if wireless LANs are to offer the same mobility that cell phone users take for granted.
802.11h attempts to add better control over transmission power and radio channel selection to 802.11a. Along with 802.11e, this could make the standard acceptable to European regulators.
802.11i deals with 802.11's most obvious weakness: security. Rather than WEP, this will be an entirely new standard based on the Advanced Encryption Standard (AES), the U.S. government's "official" encryption algorithm. The Task Group in charge hasn't yet chosen an authentication protocol: Some members want to use a new system called Offest Codebook (OCB), but this is covered by three separate patents; other members would prefer one that everyone can adopt royalty-free.
802.11j is so new that the IEEE hasn't officially formed a task group to discuss it, let alone produced a draft standard. It's supposed to cover how 802.11a and HiperLAN2 networks can coexist in the same airwaves.