Research is a tradeskill that allows non-priest casters to research and create spells through combining components. Pure casters may pick this tradeskill up at level 20.
Often, the combination of paper, pen, and two other components (such as a word or writ) will allow the spell researcher to create a spell.
The spell created does not have to be limited to the class of player researching the spell. For example, necromancers can just easily make magician spells, wizards may create druid spells, and magicians may create ranger spells. Obviously, only spells that actually exist can be created.