Different depending on your area. A jamboree is a scrimmage but not all scrimmages are jamborees.
An inter-squad scrimmage is amongst one's own team, predominantly a college thing. Here in my area, scrimmages can be two, three, four, or more teams. Jamborees are mostly youth football events, where multiple teams play either a cut down game or controlled scrimmages.
In NC, HS scrimmages are totally controlled by the home coach. He sets the format, we are there for training and reps. If they want to stay on the field, they do. Some are well organized, some are fly by night affairs where a coach can dream up whatever crazy thing he wants.