802.11a has speeds of up to 54mbs and uses a regulated frequency range that prevents signal interference from other devices but it has a fairly small range and is easily blocked by walls etc.
802.11b has speeds of up to 11mbs, uses a unregulated frequency range so can suffer from interference from other equipment has a better range than 802.11a though and is not easily blocked by walls
802.11g has speeds up to 54mbs, uses the same unregulated frequencys as 802.11b so can suffer from interference but also has the good range and is not easily blocked by walls.