A spellcaller is able to break through the bounds of reality and summon a creature or entity to do his bidding, for example, a creature from the Pits of Endless Strife, or a Shadow or Radiance entity from an alternate frequency. They are also able to do the opposite of course, banishing summoned creatures

NWN specifics Spellcalling spells are spells like the Summon Creature line of spells, Gate and Banishment spells.