Dual channel works just the way you would think it would - it provides dual channels (access paths) to RAM.
What this means is that - in modern CPUs, a memory path is 64-bits wide. Dual channel generally requires two RAM modules or more, and usually channels are color coded on the Motherboard. This means that you can install two DDR/DDR2/DDR3 RAM modules (or 'sticks) that are identical in the opposite colored slots and achieve dual channel (or 128-bit) mode. This is mode (two modules) provides the best throughput performance for the machine, although not necessarily the best performance altogether. For example, let's say you had two DDR3-1600 modules of 1GB each installed in dual-channel mode, you would get great throughput (bandwidth/transfer speed), but might suffer from a lack of total available RAM. Best option available on modern Motherboards? 2x2GB (4GB) or 2x4GB (8GB) with lowest latency for that Motherboard. Usually more RAM is better as a rule, but above more than 4GB latency becomes an issue and is only necessary if the additional RAM can be used.
Hope this helps, hit me back at vh1atomicpunk@yahoo.com if you have other questions.