We already discussed how MIMO works.  Let’s look at another technical improvement currently utilized in 802.11n – channel bonding:
channel-bonding
The graphic is fairly self explanatory – traditional 802.11 channels are either 20 MHz wide (OFDM) or 22 MHz wide (DSSS).  Channel bonding combines two adjacent channels, which effectively doubles the amount of available bandwidth.
One footnote to channel bonding is that it works best in the 5GHz frequency band, as there is only space for three traditional, non-overlapping channels in the 2.4GHz frequency band.   Therefore, there is only enough space for one bonded channel in that portion of the RF spectrum.