First a quote from car-safety.org:
The LATCH system was originally called ISOFIX, a term still used in Europe. In Canada, it is the Lower Universal Anchorage System (LUAS) or CANFIX. It has also been called the Universal Child Safety Seat System or UCSSS. Some carseat companies have trade names for this system, for example, Britax uses the term "ISOFIT™". All of these names refer to the same universal anchorages that appear on a few model year 2000 vehicles and even more model year 2001 and 2002 vehicles. These are required by law in the USA for almost all model year 2003 and later vehicles. With the exception of the Volkswagen Passat and many Audi models since 1999, lower LATCH anchors cannot be retrofitted onto earlier model vehicles.
Isofix stands for International Standards Organisation FIX.
Isofix is a system specially made for fastening children's car seats(intallation without the car seatbelt). The car seat has two pairs of "claws" that can be attached on to to anchor points behind the seat of the car(usually found in the back seat). It is supposed to be an easier way to install the car seat and to reduce the risk of installing the car seat wrong.
The anchor points are most often found in the cars back seat on the right and the left side.
The illustration below is of the European isofix system:
If the car seat is correctly installed using the cars seat belt their is no difference in the safety between a car seat installed with isofix and a car seat fastend with seat belts. But as mentioned earlier the risk of installing the car seat wrong is reduced when the seat is fastened with isofix. If one has a booster with isofix it is also safer when driving the car without the kids, beacause many parents drive around with boosters lose in their car when the kids don't sit in them. When the seat is installed using isofix one don't have to remember to fastend the seat every time one goes for a ride without the kids. Remember that a lose car seat can cause great damage in a accident, it will come flying trough the air like a projectile.
From 2007 isofix should be standard on new cars in Europe. Some car manufactures have had the isofix system in their cars years already, the first car with isofix came in 1997. Here is a list over cars with isofix(produced before 2007).