Ions are atoms which have gained or lost electrons. The ionization energy is the amount of energy it takes to detach one electron from a neutral atom.
Some elements actually have several ionization energies.
I i where i is the orbital from which the electron is lost. The IE increase from bottom to top and left to right in the periodic table. Below is a diagram showing the directions atomic size increase over the periodic table. As you can see, the IE and atomic size increase in opposite directions. This should make sense because as the atom gets smaller, the valence electrons become closer to the nucleus.