802.11a = 5GHz 54mbps
802.11b = 2.4GHz 11mbps
802.11g = 2.4GHz 54mbps
Those speeds are the max, there are slower speeds as the signal drops, the speed will drop back.
Haris is talking about a router that supports all three of those. I have a Dlink access point that supports those three. I only use 802.11a and g however. My Toshiba laptop has an 802.11a/b/g WNIC and it will connect to the Dlink AP with a speed of 108mbps. Although, of course, it's still not faster than the 100mbps wired NIC that's built into the laptop.