Periods are the rows of the periodic table. As you go along periods the number of protons and neutrons increases and the number of electron shells remains the same.
The periods of the periodic table are the rows that run from left to right in the periodic table. The elements are not arranged in these because of similar properties but because of their increasing atomic number from left to right.
The origins on the periodic table begin with the work of John Newlands and Dimitri Mendeleev.