Summon wrenches a creature's soul from the netherworld to help the caster. If this creature kills another, it is replaced by the soul of the newly fallen victim. Higher levels allow the caster to keep the creature summoned for longer.
A summon is as strong as the creature it represents.
Recasting Summon while you have a summon out is less costly than an initial cast, making it a useful spell to cast repeatedly in order to gain Will experience. However, it does not reset the summon time.
Different creatures do not cost different amounts of mana to summon.
Kills and actions by your summon do not affect you (experience or morality).