By law all car seats sold in Europe must meet standard ECE R44/04 (or 03). This standard groups car seats by the child's weight. In July 2013 an new car seat regulation was introduced. This regulation is called R129 or i-Size.
I-Size will eventually replace the current standard ECE R44/04. It is different as it groups car seats by size as well as weight, a bit like clothes sizes.
Two of the most important changes are the addition of a side-impact test which R44/04 does not include, and rear facing to a minimum age of 15 months. Eventually all new cars will have two i-Size compatible seating positions, which will be a guaranteed fit for an i-Size car seat without the need for a compatibility list.
The first phase of i-Size covers Isofix seats. The next two phases of i-Size will cover belt fitted car seats and booster seats. They will be phased in by 2018 when seats that meet standard ECE R44/04 will stop being produced. Although you will no longer be able to buy them, existing R44/04 seats will still be legal to use for several years.
In an i-Size car seat children have to be rear facing until they are at least 15 months old regardless of their weight and size. Under R44's weight group system, forward facing is legal from 9kg regardless of the baby's age. Whether you choose to buy an i-Size car seat or an R44 one, rear facing up to at least four years old is always safer.