Groups on the periodic table are the vertical columns. Sometimes groups on the periodic table are called families. Main group elements that are in the same group on the periodic table share similar chemical characteristics because they have the same number of electrons within their highest energy levels. In other words, elements that are the same group on the period table have the same number of valance electrons. In fact, the number that resides over a main group on the periodic table correlates to the number of valence electrons that the elements within that group have. For example, the first group of elements have one valence electrons. Valance electrons predict reactivity because they are the electrons that are involved in creating chemical bonds.  

Please note that there are exceptions to this rule such as the transition metals, hydrogen, and helium.