802.11n is the next evolution in Wi-Fi communications, and is the first dual-band standard from the IEEE. 802.11n will leverage much of what has been learned over the years from 802.11a/b/g, including the latest developments in QoS, power saving, MIMO (multiple-input-multiple-output), and leaves the door open for future technologies such as beam-forming.
802.11g works in the 2.4 GHz band only, supports a single data stream between the AP and the mobile user device, has a maximum "link rate" of 54 Mb/s, and a maximum throughput in the range of 20-26 Mb/s. 802.11g is also susceptible to something called "multipath fading", where as the signal reflects off of the walls, ceilings, furniture, etc., it can create dead spots, also known as "nulls." You'll notice that most 802.11g access points have two antennas, which uses a technology called "diversity" to help overcome the effects of multipath. 2.4 GHz communications is also affected by microwave ovens, Bluetooth devices, 2.4 GHz cordless phones, and other 2.4 GHz neighboring wireless devices such as other 802.11g and 802.11b APs and routers.
802.11n will operate in both the 2.4 GHz and 5 GHz bands, and supports multiple simultaneous data streams between the AP and the mobile user. These are also referred to as "spatial streams," as they utilize the multipath I mentioned above to "multiplex:" multiple individual data streams for a much higher total aggregate throughput. You'll see references to 802.11n eventually supporting upwards of 200 to 600 Mb/s throughput, depending upon the number of simultaneous spatial streams. As you'll agree, this is quite a bit more than the throughput of existing 802.11g or 802.11a products. In this way, 802.11n actually benefits from multipath, where 802.11a/b/g suffer from it. 802.11n will also provide a high-definition video mode at 5 GHz, ideal for transporting multiple high-definition video streams between your home theater devices, cable modem, camcorder, DVD player, etc. Lastly, as 802.11n supports the 5 GHz band, it will provide many more channels to choose from, less chance of interference from neighboring networks, and be immune to the negative affects of microwave ovens, Bluetooth, etc.
Today you'll see "Pre-N" or non-standard home versions. The reality is that it will be mid-to-late 2007 before 802.11n is ratified as a standard and then tested for interoperability.