There are some places where you definitely don't want to drink the water that comes out of the tap in your hotel room, and maybe even rinsing your mouth after brushing is not such a great idea. Bacteria, viruses, protozoa and intestinal worms are some of the pathogens and parasites that can live in water, and they're a potential hazard for travellers in the Third World in particular.
If you don't have sterile water or if your hotel doesn't provide it, there's a quick and easy solution provided you have an electric kettle in your room. Common intestinal pathogens are rendered inactive by heat. Some of these pathogens will die at temperatures as low as 60°C. By the time water reaches boiling point the threat from waterborne pathogens is eliminated and the water is safe to drink.
The US Centres for Disease Control recommends boiling for one minute. Since water boils at a lower temperature as altitude increases, boiling water for three minutes at altitudes of 2000 metres and above is recommended.
While boiling water can eliminate the threat from waterborne pathogens, it will not remove chemical toxins. Also, some pathogens can survive freezing temperatures, and ice and ice-cream can be contaminated.
If you're eating out and have doubts about the water served on the table, ask for water with gas, fizzy water or whatever the local term is for carbonated water, chances are it's safer.