The 802.11a spec is the technology upgrade to 802.11b .  802.11a operates in the 5Ghz frequency range and can transfer data at the rate of 54Mbps!  That is over 4X the transfer rate of 11Mbps 802.11a.  Although it is another form of Wireless Ethernet, 802.11a is incompatible with 802.11b products.  They are completely different technologies.

802.11a network adapters can operate in two modes, infrastructure and ad-hoc.  In infrastructure mode, all network adapters talk to each other THROUGH a central 'Access Point'.  This access point grants permission to each device, determines the frequency to communicate on, and relays data between network adapters.   Access points are stand alone devices that can 'bridge' wireless computers to wired ethernet computers.  They also may be built into other devices like network routers - especially in the consumer market.

Since 802.11a products communicate in the 5Ghz frequency range, they do not cause problems with cordless phones like most 802.11b products do.  Unfortunately, 802.11a also has a range limitation in comparison to 802.11b - often 1/3 the range.