Well water is extracted ground water on private land. According to EPA 78 percent of Americans get their public water source from ground water. The main difference is that public tap water is regulated by the EPA and controlled by the city while it is up to the owner of the well to maintain that water's purity.
According to the CDC, ground water accounts for 30 percent of all avaliable fresh water on earth. Most people who use tap water in the U.S. get their water from underground sources. Personal wells also take out water from the ground water reserves. Other water sources include rivers and lakes which only account for 22 percent of American consumption, according to the EPA.
Tap water is regulated by the EPA to uphold safe standards and prevent water pollution. It is up to the city to uphold these standards and test the water regularly to make sure it is safe to drink. Wells, however, are not regulated by the EPA and it is up to well's owner to maintain it and make sure it is safe.
Tap water is safe to drink because of regulations and testing. Well water can be contaminated if not properly maintained. Well water can be just as safe to drink if the owner does testing and inspects the well cap regularly. Professionals can be hired to make sure the water is safe.