What is this plant?

It's a Kalanchoe and a succulent. It prefers a west or east facing site during the summer then a move to facing south in the winter. As for watering, it doesn't like sitting in wet compost, so water it when the compost is on the dry side. Good luck!

Thanks very much! It's on a west facing windowsill at the moment, but we don't have any south facing windows so I hope it will still be ok there in the winter.

Spritz is right. The common name is flaming katie. If you want to make more, just remove a leaf and pust the tip into a pot of compost and it will root really easily. However, the baby wont grow as big as Mum, not without the very specific nursery conditions they need to get big.