There are three types of N standard routers.
1) Single Band: These are at the bottom end of the spectrum. Single band means that the router only broadcasts signals on a single frequency, i.e., 2.4Ghz. The budget N routers are usually single band routers.
2) Dual Band: Dual band routers can broadcast a signal on EITHER the 2.4Ghz or the 5Ghz frequency. So there will be just a single wireless network, but it's up to you whether it's the 2.4Ghz network or the 5Ghz network.
3) Simultaneous Dual Band: These are at the top end of the spectrum. These routers can broadcast signals on both frequencies, 2.4Ghz and 5Ghz, simultaneously. So if you have a simultaneous dual band N router, you'll effectively have two wireless networks. One on the 2.4Ghz frequency and one on the 5Ghz frequency.
The range of the 5Ghz network is typically less than that of the 2.4Ghz network, however, the throughput is better due to less interference and stuff.