Each conjuration spell belongs to one of five subschools. Conjurations bring manifestations of objects, creatures, or some form of energy to the spellcaster (summoning) actually transport creatures from another plane of existence to the spellcaster's plane (calling), heal (healing) transport creatures or objects over great distances (teleportation), or create objects or effects on the spot (creation).
Creatures conjured, in NWN2, always obey the spellcaster's basic commands, although cannot be possessed by puppet mode.
For a list of all spells which are of the school of Conjuration, see: Category:Conjuration spells.
Conjuration spells are, of course, geared towards summoning and conjuring magic. You will, of course, get access to all the Summon Monster spells, as well as Planar Binding and Gate. As a Cleric or other wisdom-based caster, Conjuration means you have access to all the healing and restoring spells, such as Heal, Raise Dead and Restoration.
If you are an arcane spellcaster and dislike summons, this generally what the school is - however, there are other useful spells such as Mage Armor and Improved Mage Armor and others for defence, as well as Acid Fog and Vitriolic Sphere and others for causing damage.
Subschools are absent from NWN2, although they are generally only descriptive of what the spell performs.