Try this explanation:
802.11a - the 1st generation, frequency - 5 GHz, max. data speed is 54 Mbits/sec.
802.11b - the 2d generation, frequency - 2.4 GHz, max. data speed is 11 MBits/sec.
802.11n - the last generation, frequency are - 2.4 & 5 GHz, max. data speed is 150 MBits/sec. and 600 MBits/sec. Supports 802.11a and 802.11b standarts.
