What's the difference between a Ring Group and a Queue?

A ring group is simply an extension that has call forwarding enabled to one or more phones at the same time. A queue, on the other hand, is a specialized extension that is purposefully designed to dispatch calls to several phones.
A queue also has the ability to keep people on hold for a determined amount of time if all phones that are in the queue (referred to as representatives) are busy on other calls.
Compared to a ring group, a queue has many advantages: queues provide detailed statistics (holding times, abandonment rate, etc.), queues are smart as to not follow incoming call rules when dispatching calls, representatives in the queue can use a feature code to log in and out of the queue to control if they receive calls or not and queues group the call recordings (if enabled) of all calls underneath a single place.
While there are a few rare exceptions, we generally recommend using a queue whenever you want to ring more than one phone at once.