A roll cage is a metal framework that can be built around the inside of a vehicle.
Building a roll cage 101.
Adding diagonal main hoop support braces turns it into and 8 point roll bar.
Then you build upon that by adding bars for an 8 point roll cage then progress to an 8 50 or 10 point roll cage.
The basic 4 point roll bar consists of a main hoop behind the driver two rear struts and an optional cross brace on the main hoop should you need it or the rules.
This is the typical 6 or 5 without the passenger door bar nhra style roll bar.
The bars shown in blue are optional.
Shown here is a visual difference between your basic 4 point roll cage and an 8 point roll cage from the chris alston s chassisworks catalog.