Can someone please clarify the difference between a bag and a quilt?

I get that there are differences in design, but it seems that when it comes to quality and construction, there are plenty of top notch products.

I'm a ground sleeper, no time for hammocks here. How do I learn if I'm to be better served by a bag vs a quilt?

I ask, because for some reason I'm drawn to the JRB site, but I'm seriously not sure why.