After transferring an electron, the ion that lost an electron, assuming it was neutral before, would be a cation. The atom that gained an electron would be an anion.
The ion is negatively charged because it has gained an ion, adding a negative charge to the atom's neutral state. Because it has no protons to cancel the charge, it gains 1 negative charge, making it an anion.
The symbol for a negative ion would be (-) and a positive would be (+)
For example, when oxygen is an ion, here's how it would look: O2-
