There are really only two types of leaky pipes: bad and worse. Well, actually small leaks aren't that bad to stop until you can replace the pipe. If you have turned off all the water inside and outside your home but you can still hear it running, check your water meter. If it is flooding, water is leaking from somewhere.
Look for the leak wherever pipes are accessible to view (ie. under sinks, toilets etc). If you can't see a leak, look at your floors, walls, and ceilings for any water damage. Stains are a sign that water is leaking down between plaster, drywall, or floorboards. Also check out the garage, basement, or crawlspace, because there may be pipes running through them.
If you think a concealed pipe is leaking, turn off the main water supply to the house. You should get a professional in as soon as you can to look at the problem. Leaks that are hidden in the ceiling, floors, or walls can cause mold and serious structural damage.
If it is an exposed pipe that's leaking, try fixing it yourself, or at least patch it up while waiting for the professionals.
Tip: If you're working on pipes with hot water, let them cool down first. They can burn you.
Here we address other pipe problems: