Gravity fed. If you have a gravity fed tank system as well as the boiler you'll also have a hot water cylinder in a cupboard somewhere and also much higher up (like in a loft) a cold water feed/expansion tank(s).
With these systems you will usually only have mains pressure to your cold water taps, so turning off your mains will only shut these off. The hot water taps will be fed from the tank in the loft which flows by gravity down to the hot water cylinder and forces hot water out to the hot water taps.
Some systems also feed cold water to bathroom cold taps and the toilet cistern so the only mains water is going to the cold tap in the kitchen.
If you are doing work on say the hot water tap in the bathroom you need to turn off the mains first, then you'll have to run the tap until the tank in the loft empties. You don't have to do this if you can find a stop cock coming from this tank. Simply turning this off will stop the flow of water. This is actually much more preferable because if you do have to drain the tank you may end up with an air lock in the pipes which can sometimes be a bit tricky to get rid of and means you'll get very little water coming out of the tap!
Some systems incorporate a pump to help provide greater pressure at the taps. In residential areas with low water pressure a gravity or gravity/pumped system is usually the best option.
How to turn hot water off : to turn off hot water locate the isolating valve(stop cock)and turn that off. It will be located on a pipe coming out of your cold water feed tank(in the loft or a high up space), OR, you may have an islolating valve on the pipe coming out of the top of your hot water tank.
How to turn cold water off : to turn off cold water locate your mains incoming supply and turn that off.
NB this may only turn off some cold water taps. In the bathroom and toilet the cold supply may come from the cold water feed tank. Locate the isolating valve(stop cock) coming out of the feed tank and turn that off