b = 11Mbps, 2.4Ghz band, range = 100-150Ft
g = 54Mbps 2.4Ghz band, range = 100-150Ft
a = 54Mbps 5Ghz band (currently no public hotspot), range = 25-75 feet
and A is incompatible with B and/or G. while B and G and compatible.
the range i mentioned is typical Indoor range.
A will might not flourish as the way B and G did, and as well you'll find very very less or even NO hot spots for A standard. while its more likely that exsisting B standard hotspots will convert into G soon, however both are compatible to each other (means you can run a G standard wireless card on B stabdard Wireless gateway, and vice verca).
hope A,B and G standards are cleared well.