The Pauli Exclusion Principle states that no two electrons can have the same four quantum numbers. The fourth quantum number is the electron spin quantum number #m_s=+-1/2#. An orbital can contain a maximum of two electrons, which can have three quantum numbers in common, but not the same spin quantum number. One must spin up, #+1/2#, and the other must spin down, #-1/2#. We represent the electron and its spin with up arrows and down arrows, #uarr# and #darr#, in an orbital diagram.
Source: http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Electronic_Configurations/Pauli_Exclusion_Principle
The #1"s"# orbital can contain a maximum of two electrons. In the diagram below, notice that hydrogen's single electron is represented by an up arrow. Helium has two #1"s"# electrons. The first is represented as an up arrow and the second is represented by a down arrow. The up and down arrows represent opposite spins.
