When the positive and negative charges in an atom no longer balance, the atom is called an ion. A cation is an ion with more positive charges than negative charges. An anion is an ion with more negative charges than positive charges.
It is because it has positive charges (from 2 H) and negative charges (from O), that the charges balance out (negative + positive = neutral) (2H+) + (O2-) -----> H2O
There are two types of Electric charge that hold an atom together: Positive and Negative. If the two charges are opposites (e.g. Positive and Negative), then they will attract. Hence, like charges will repel and go separate ways.
If an atom has 3 positive charges (protons) and 4 negative charges (electrons), the 3 positive charges would "cancel out" 3 negative charges, with one negative charge left over. So the atom would have a charge of -1.
If the atoms have opposite charges (positive to negative) they will attract. If the atoms have the same charges (positive to positive or negative to negative) then they will repel. You can look at the Law of Electric Charges to get more information on this.