Dual band uses both the standard 2.4ghz range + a 5ghz for the second band. A true dual band has two separate radios capable of running 1 client with 2.4ghz b/g/n and 1 running 5ghz a/n. However, while the speeds are faster with the dual band, the range may be considerably lower. If you need the range, 2.4ghz goes much further. If you need the throughput, the 5ghz can be marginally faster. It's not for me, but for some people its great.
www.smallnetbuilder.com has some router tests that may interest you.