Chemically speaking, water is simply H2O, two hydrogen atoms and one oxygen atom. Here's a picture of it:
However, when you drink water, it's not just these humble molecules floating around in your glass. In fact, if your water wre truly just water, you would die of water intoxication!
There are also tiny amounts of salts (which in the right amounts make water taste good), and depending on your city, there might also be trace amounts of pollutants like unhealthy minerals (lead, excess of fluoride salts, and so on), organic chemicals, and hormones which could make you sick over long periods of time. While I try to steer away from anything political, I would recommend checking out this report by the Environmental Working group to see if your city is on there and how it ranks. Also, many cities publish their water quality data every year, and you can search for it online.
If you want to know more about how safe your water is to drink, or what some of the chemicals in water are, feel free to comment or ask a new question!