What does it bring to the party that's new? Well, 802.11n builds on the 802.11 spec by allowing for a new feature called Multiple Input/Multiple Output (MIMO). MIMO uses multiple transmitter and receiver antennas to improve system performance. To further enhance its capabilities over a legacy router (a/b/g), the new 802.11n spec uses the 5GHz spectrum to reduce the interference issues found on 802.11g routers using the 2.4GHz spectrum.
For the past few years, the IEEE Standards Association -- the wireless spec governing body -- has issued a number of upgrades to the N spec, from pre-N to Draft-N to its current stage of development: Draft 2.0. Generally speaking, newer routers work on this new spec, which is the result of thousands of minute improvements to previous iterations. It's also worth noting that 802.11n has not been approved by the IEEE and should still be considered a work in progress.
Currently, there are two key questions to ask before purchasing any specific 802.11n router: Is it worth buying? And does it perform well enough to justify junking your 802.11g router and spending money on the new device?