Dual channel memory is a somewhat simple concept but kind of complicated in execution, but lets start with what DDR is.
Double Data Rate memory describes memory that writes and reads on the rising and falling edge of the clock cycle pulse (a square wave).
800Mhz clock equals DDR 1600 as an example. DDR2, DDR3, each describe a generation of hardware. With each successive generation power consumption, ie temperatures, go down along with voltage. This allows for higher clock speeds.
Dual channel memory is the process of taking the same information and writing to both RAM modules simultaneously. In theory this can double bandwidth. In practice, not all data is the same, so it will take an extra cycle to write the different data.
If a 1 needs to be written to module 1, and a 1 needs to be written to module 2 it will do it at the same time. If they do not match is when it will need the extra cycle.
This is why memory is now commonly sold in matched sets. Dual, Triple, and Quad kits being available. Dual channel for modern consumer class motherboards. Triple channel for X58 professional class boards, and Quad channel for current X79 professional class boards. I believe Haswell-E is slated to introduce DDR4 quad channel sometime in 2014-2015.