As you've experienced, when you turn the faucet on, the water comes out with some force. That's due to the pressure contained within the piping system. Firstly, the water is drawn into an intake, let's say from the river. It then flows to a treatment plant to take out the impurities. It then either flows or is pumped into the distribution system... a series of pipes that gradually descend in size. From over a few feet in diameter down to your house service, which is about an inch in diameter in most cases. The pressure is always in the pipes which makes the pressure available at your tap. The water comes into your house and part of it goes to the hot water heater, which is then distributed to various places in the house. The other pipe provides cold water which is cold because it is coming from fairly deep in the ground where the pipes are buried... and voila! This is greatly simplified and there are several different types of storage and distribution, but the over all system is the same...