Control Group? What is it?

Sometimes when I group objects, the group becomes a "Control Group."

1.) What is a control group?

2.) Why does a group become a "control group" when I didn't specify it as a control group?

3.) How do I ungroup objects in a "control group?" When I try to ungroup "control group" objects, nothing happens -- all the objects remain as a "control group."

Tks.