In several countries, the local tap water is declared "non-potable", meaning that it contains bacteria, viruses or parasites that make it unfit for human consumption. Drinking contaminated water could, at the very least, result in a nasty case of "Travellers' Diarrhoea", something suffered by 20-60% of overseas travellers, so it's definitely worth a little forward-thinking to ensure your holiday experience is a healthy and positive one!
Probably the easiest way to avoid infection from waterbourne diseases is to stick to drinking bottled water, but beware! It's been known for people to sell tap water in old bottles, so when buying bottled water ensure it's sealed and is from a recognised brand and if you want to be 100% sure, choose fizzy water rather than still. Where possible, use tap water when brushing your teeth too and avoid ice in drinks or any uncooked fruit or veg, unless it's been peeled first. Fresh fruit juice is another great alternative to tap water, and a good source of fluids for children who dislike drinking plain water.
Another option where tap water is unsafe to drink is boiling, which works by killing off any disease-causing agents which may be lurking in the water. To do this, make sure there is no visible dirt or any foreign objects in the water before bringing it to a rolling boil for at least 5 minutes in a pot or kettle. Remember to ensure that the container you're using for storage is sterile too! If you prefer to use a Water Purification System, keep it in your backpack at all times so that it's always on hand when needed.
Specific information regarding sanitation levels in the country you're travelling to can be obtained from the National Travel Health Network and Centre via their website, which can be found at http://www.nathnac.org/travel/index.htm.