This is the first time it's been possible to fill in the UK Census online. It's held on the same day - March 27 - throughout the UK, with different paper forms for England and Wales, Scotland , and Northern Ireland. These forms should be destroyed if the information is given online.
Strictly this is a "snapshot" of the population as it stands on this particular day, so only fill it in before 27 March if you're sure nothing will change. It's a legal requirement and anyone trying to avoid taking part can be prosecuted and fined up to 1000 GBP.
About 200 countries in the world regularly carry out population censuses. They are supposed to provide an insight into our lifestyles and needs.