ISOFIX was introduced as a child seat standard during the 1990's. In Europe, all new cars must be fitted with ISOFIX fittings, which are in effect metal clamps which are housed (concealed) within the rear seats of the car. ISOFIX seats have specific fittings which attach to the ISOFIX hooks in your car. If you don't have an ISOFIX seat, you'll have to rely on the rear seat belts to install your child seat.
The great advantage of ISOFIX seats is that they are easy to install and you can be pretty sure that you've done it correctly. If you need to take your seat out of your car regularly and have difficulty putting it back correctly, then ISOFIX is a good idea. Some German studies recently showed that only 30% of non-ISOFIX child seats were correctly fitted, but 96% of ISOFIX seats were fitted in the right way. With all seats, make sure you're shown how to correctly install the seat when you purchase - most reputable sellers will help you with this for free.
The ISOFIX system can also reduce the forward and sideward movement of the seat and limits rotation through a foot prop or top tether (universal or semi-universal seats). In some of our tests, we found some ISOFIX seats to be heavy and bulky, with the child pushed forward towards the back of the passenger seat. Try to take into account the weight and positioning of the seat before you purchase, particularly if you have a small car or a child with long legs!