Water can be turned off from from inside and outside your house. It's a good idea to familiarise yourself with where both your stopcocks are before there's a great spouting leak causing irreparable damage. The inside one is usually found under your sink in the kitchen (however, if there's one truth about DIY it's that NOTHING IS EVER CERTAIN, so although it's likely to be located there, it may not be). You may have to remove a kickboard to find it.
You may find that you have two stopcocks next to each other. One will not work, and is likely to have been left behind from a former plumbing job. Test to find out which one works by turning them on and off. To close a stopcock, simply turn it clockwise.
Caring for your stopcock
Stopcocks aren't just for Christmas. Give yours a spray of WD40 every now and again, and keep checking it. They need some TLC as they can become quite brittle over time, and may not be easy to turn off and on.
If you can't find your stopcock indoors then you definitely need to know where you can turn the water off from the outside. Go outside and look on the pavement for a smallish manhole. It should have 'water meter' or the name of the company that provides your water on it. Get a flathead screwdriver (not your best new one) and prise it up. Underneath should be a dial. It might be encased in a polystyrene or plastic case. If so, open it up to see the dial.
There could be more than one (for instance, if you live in a flat or converted house). If you're not sure which is yours then for God's sake don't turn anything off. Go into your house and turn on the tap full whack. Come back outside and see which dial is jittering about. The moving one is yours. If more than one is moving, you're an unlucky person and need to try again later.
Your outside stopcock may need a key. Often it's kept in the casing or somewhere within the hole your meter sits in. However, it might have been lost or the stopcock could be a simple tap sort, like the indoor one. Whatever you're faced with, it's better to know about it (and deal with it) before you need to know. Bear in mind that your street one belongs to the water authority and if you break it, it's your responsibility.
Find out which water system you have
In a direct water system, your water is fed directly from the mains water. In an indirect water system, the water is fed from a water tank. You are likely to have a cold water storage tank somewhere in your house (probably in the attic).