1.28 describe the formation of ions by the gain or loss of electrons

An ion is any atom (or group of atoms) that has charge / is electrically charged. Ions are formed due to the loss or gain of electrons.

If an atom gains an electron it becomes a negatively charged ion. Non-metals tend to do this, and they form anions. So elements from group 5-7 will form anions.

If an atom loses an electron it becomes a positively charged ion.the ion has a positive charge. Metals tend to do this, and they form cations (positive ions), so normally elements from group 1-3 will form cations. 

Group 0/8 are the noble gases and are inert + unreactive, so they do not form ions.

NOTE: an easy way to remember anion is A-Negative-ION - ANIONAn easy way to remember a cation is that if its not an anion, its a cation :)