A group on the periodic table is one of the vertical columns found on the table. The elements found in the same group all possess the same number of electrons in their outer orbital shell. These electrons are specifically known as valence electrons.
On the periodic table, there are 18 groups. A group is also known as the family that the element belongs to. There are multiple ways of labeling each group. One way is simply to number each group from left to right through 18. Another way is to name the topmost element of the group, such as the oxygen group.
Valence electrons are used when one element chemically bonds with other elements.