What causes ionic bonding?

[Pengwuino]

So I never got too far into chemistry and had never had much thought put into this. What exactly causes an ionic bond? And I mean, I know the usual 'one atom releases an electron and another gains it, causing net electric charges on both atoms', but what causes the electron to leave an atom to join another?

For example, a sodium atom coming near a chlorine atom. What causes the valence electron to disassociate with the sodium atom in the first place? A stronger dipole moment on the chlorine? magic? :)

[mishrashubham]

This has to be explained thermodynamically. I find this site (chemed.chem.purdue.edu/genchem/topicreview/bp/ch7/whydoes.html) pretty good.