The Dev Channel is largely similar to the Fast Ring, it's pitched as a way for highly technical users to get early access to the latest Windows 10 features. The downside is that Dev Channel releases will have some instability as these contain the highest number of bugs.
The Beta Channel is the new version of the Slow Ring, it's designed for early adopters and will feature more reliable builds. Then there's also the Release Preview Channel, which is a new name for the Release Preview ring. The Release Preview Channel gives access to upcoming, supported releases of Windows 10. More details at the Windows Blog.
For Windows Insiders, the move to channels will be easy. If your device is currently flighting, we’ll automatically move your devices based on your current ring to the new channel that best corresponds with that experience. You won’t have to take any action for this change to happen.
Insiders can always adjust these settings by going to Settings > Update & Security > Windows Insider Program if they want to change channels and try out new experiences.