ISOFIX is an international standard of fitting child car seats. In most modern cars, and even some older cars, there are ‘D’ shaped hooks in the base of the back seats on either side (some vehicles may have ISOFIX points in the middle or front seat) to find out if you have ISOFIX, either put your hand in the join between the passenger seat base and the back and run your hand along it until you feel them, or check your manual.
ISOFIX is considered safer than a seat belt fitted child seat as the risk of fitting the seat incorrectly is very small. An ISOFIX child seat is also quicker to fit, easier to fit and is attached to the chassis of the vehicle.
ISOFIX seats also come with other safety features like an impact leg or a top tether. These devices must be used, if supplied, as they reduce the rotational and forward movement of the child and the seat in an impact.