802.11g and 802.11n differences

802.11g
2003 – Speed 54Mbit – Operates in 2.4 Ghz band – One can have 3 non overlapping channels (channel 1, 6 and 11)

802.11n
2009 – Up to speeds of 300 Mbit – Operates in 2.4 Ghz and 5 Ghz band
If 2.4 Ghz used still have 3 non overlapping channels (channel 1, 6 and 11)
If 5 Ghz used still have 23 non overlapping channels

The n draft uses the MIMO technique, this means that the normal 20Mhz (150Mbps) channel is doubled to 40 Mhz (300Mbps). This doesn’t mean that you’ll have 2 times the number of non overlapping channels. It assures twice the data transfer rate due to multiple antennas on your access points.