According to the Beverage Marketing Association, a trade group, nearly 50% of all bottled water sold in the U.S. is just tap water that's been purified. The regulated standards for bottled water and tap water are actually very similar. The biggest difference is that bottled water is produced specifically for "human consumption."
But with that said, tap water is regulated by the Environmental Protection Agency (EPA) to ensure that it's safe to drink, and many experts say tap water is actually regulated more tightly than bottled water. There's just no fancy label on your faucet. Plus, tap water typically contains fluoride, which helps maintain strong teeth and prevent tooth decay, and fluoride is often removed from bottled water during the purification process. Mored info here.