The Period (or horizontal rows) in the Periodic Table
The Period number indicates the energy level where the valence (or outermost) electrons are located. An element found in the 4th Period has its valence electrons in the 4th energy level

The Groups/Families (or vertical columns) in the Periodic Table
Elements which are found in the same column of the Periodic Table have similar valence electron structure. Because of this they also have similar chemical and physical properties. For instance, the Halogens (Group 17) all have 7 valence electrons