Binding is the process by which a divine or arcane spellcaster procures the services of an outsider, a creature of the Great Beyond. The ability to control such creatures is one of the most powerful tools a spellcaster has in her arsenal. The outsider must first be called from its native plane by a powerful spell, and then convinced to enter the spellcaster's service for a defined amount of time. The method by which this is done depends greatly on the type of spellcaster performing the binding and the creature that she is attempting to control.
Divine spellcasters have the easiest time binding outsiders, but are limited in the types of creatures they can bring into their service. They generally accomplish this via the planar ally family of spells that call a creature allied with the caster's deity or one that shares the caster's general philosophy.
Arcane spellcasters have a much more difficult time binding creatures than their divine spellcasting counterparts, but are only limited by their own skill in the types of creatures they can call and bind. They must first prepare an inward facing magic circle to trap the outsider before calling it from its plane via the planar binding spell.