An ion is an atom or molecule that has more or less electrons than protons, as opposed to a neutral atom in which the numbers match. Because of this, the ion has an electric charge, either positive or negative and are subject Coulomb force, i.e., in the presence of an electron, positron, or another ion, are subject to a force drawing them together (if oppositely charged) or pushing them apart (if both are positive or both are negative).
An anion is a negatively charged ion, i.e., with one or more extra electrons, and a cation is a positively charged ion, with fewer electrons than protons.
Some molecules are essentially ions bound together by the Coulomb force, i.e., an ionic bond (whereas others are bound through more complicated details of small-scale electromagnetism, i.e., its quantum-mechanical nature).
Sufficiently high temperatures tend to dissociate molecules, and at a higher temperature, electrons being prodded away from nuclei as quickly as they can they recombine, and the material consists of ions, and is termed plasma. By far, the majority of the (non-dark, i.e., baryonic) matter in the universe is in this state, given that the interior of stars as well as much of the interstellar medium and intracluster medium is sufficiently hot.