A leak is a small problem for as long as it takes to close a valve, and a large one after that.
The valves in a house
The street or curb stop, outside at the property line, operated by the utility with a key. Not the household's to use in most jurisdictions.
The main shutoff, where the supply enters the building — a basement, a crawl space, a garage or a utility closet. On a well, at the pressure tank.
Branch or floor valves, isolating parts of the system where they exist.
Fixture stops, under each basin and toilet and at each appliance.
The water heater's cold inlet valve.
Hose bib shutoffs, where the outside taps are separately valved.
The two kinds
Gate valves — a wedge lowered into the flow, operated by a multi-turn wheel. Traditional, and they seize when left in one position for years. Attempting to close one that has not moved in a decade frequently breaks the stem or leaves it half closed and leaking.
Ball valves — a quarter turn of a lever. They do not seize in the same way, they close positively, and the lever's position shows at a glance whether the valve is open or shut.
Replace gate valves with ball valves as they are encountered. It is an inexpensive job and it converts a valve that might work into one that will.
Exercising them
Any valve left untouched for years is a valve that may not operate.
Once a year, close and reopen the main and each fixture stop. It takes fifteen minutes for a whole house and it finds the ones that have seized while there is time to replace them.
Some seized valves will leak from the stem after being moved, which is a small repair discovered deliberately rather than a large one discovered at 2am.
Fixture stops
The small valves under basins and toilets. Frequently multi-turn compression stops in older houses, and frequently seized.
They are the valves that matter for an ordinary repair, and replacing them with quarter-turn ones during any bathroom work is cheap and worthwhile.
Braided supply hoses connect them to the fixture, and those hoses are the most common source of a serious indoor leak. Replace them on a schedule, and prefer stainless braided over plain rubber.
Everyone in the house should know
Where the main is. What it looks like. Which way it turns.
That is a two-minute exercise and it is the difference between a wet floor and a wet house. Label it, and keep whatever tool is needed beside it.
Automatic shutoff
A leak detector with a motorised valve on the main closes the supply when it senses water or an abnormal flow pattern.
They have become inexpensive, and for a house left empty for periods, or with an upstairs laundry, or with a water heater above a finished space, they are one of the better small investments available.
Point-of-use versions — a sensor and a valve at the washing machine or under a sink — do the same job for one appliance.
Winterising
For a property left unheated, closing the main is only the first step: the system is drained, the water heater is drained, the traps are protected with antifreeze, and any outside lines are cleared.
Closing a valve and leaving water in the pipes achieves very little, and it is worth having the procedure written down for a property that sits empty.
Access
A main shutoff behind stored boxes, inside a finished cupboard, or under a worktop is a valve that will not be reached quickly.
Keeping the space in front of it clear is free, and it is the same principle as the working clearance in front of an electrical panel — the rule exists for the worst moment rather than the ordinary one.