What is the difference between a "dual-band" and regular 2.4ghz wireless router?

Well, I was persuaded to buy an Apple Airport Extreme dual-band—to go with my new iMac (one by one I’m migrating from PCs). Like Belkin, D-Link, & other brands it can communicate at 2.4 GHz and 5 GHz simultaneously. I’m only using it at 2.4 because it’s in a centrally located room of my house & I get sufficiently wide penetration.

At 5 gigahertz there’s obviously more than twice the bandwidth, but with a wavelength of around 2–½ inches (compared to 5 inches at 2.4GHz) probably doesn’t broadcast as well. My house is old enough to have plaster & lath walls framed with with oversize lumber.

So much for theory. I haven’t done the experiment (i.e., tried 5ghz wireless in my house) because it’s a pain to set up a new network & now’s not a good time for a network outage.
I plan to eventually use the dual-band router to transmit 2.4ghz wireless to other computers in the house, while using line-of-sight 5ghz to communicate with a home theater system in the same room. Some day.

But I’m a little fuzzy myself on the intended purpose of a dual-band wireless router.

Regular wireless routers operate in the 2.4ghz frequency. So do many other devices such as bluetooth headsets and cordless phones. As the amount of devices in the 2.4ghz frequency increase so do your chances of interference on your network. Living in an apartment building with many wireless routers could cause problems for example.

A dual-band router works in the 2.4ghz and the 5ghz frequency (hence the dual in dual-band). The 5ghz frequency is newer and significantly less crowded with devices. Less devices using the frequency means less chance of interference and a more stable network.

I’m not sure if your specific router needs any manual setup, you will have to check your manual or tell us the exact model.

To continue off of @cawlin‘s explanation:

The 2.4GHz band corresponds to the 802.11g specification of wi-fi. The vast majority of wi-fi enabled products in use today support that spec. It’s also subject to interference – translate as “slower performance” – due to cordless phones, microwave ovens, etc.

The 5GHz band corresponds to the 802.11n specification of wi-fi. It’s improves on the older g (and b and a) spec by offering substantially faster network speeds. The newer products coming to market are more likely to offer support for n now.

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A device that supports 802.11n is also fully backward compatible with g, b, and a.

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A dual-band router can be configured to support two networks simultaneously… one for n (at 5GHz) and one for g (at 2.4GHz) – you typically even give them different SSID names. This is useful, for example, if you’ve got a new laptop computer that can take advantage of n but when you also want to support older devices that still only work on g (like a Nintendo Wii).

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Neither the Apple Time Capsule nor the Airport Extreme come out of the box with dual-band enabled. You have to turn it on at setup using the Airport Utility. Other brand routers likely have different factory defaults. If you can provide the make/model of your router, we can provide more details.