The formula for the sulfate ion is SO42-. The polyatomic ion is also just referred to as sulfate or sulphate.
An ion is an atom or a collection of atoms that collectively have a positive or negative charge. They gain positive or negative charges by donating or accepting electrons. Those charges help to attract other ions, which eventually form bonds, called ionic bonds.
Sodium chloride, or table salt, is another example of an ionic bond. The sulfate ion is actually a collection of atoms, sulfur and oxygen. The sulfate ion has two negative ends, meaning it donates an electron on each side from the oxygen atoms.