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.