As mentioned in the post about psionics, Dave Kenzer has said he's done a lot of work on Illusionists. As such, here's the version I came up with before he said that.
Illusionists supplement basic mage spellcasting with a powerful, flexible ability to create illusions… false images of light, sound, smell, even the perceptions of touch, taste, and heat… that fool the senses.
There are two basic types of illusions, with some very key differences. Mirages, also called holograms or, more confusingly, “true illusions” create and manipulate actual light, smells, and sounds for all to hear; a holographic thunderclap can be heard for miles, and might have people checking the skies briefly to see if rain is coming. It is not necessary for someone to believe a mirage for it to have an effect on them; even if you know your opponent is an illusionist, you are still being buffeted by light and sound; they simply do not have the sources you may be led to believe. Targets may realize that the image they are being shown is false, but that does not dismiss the image; an Illusionary Mural that you can’t see through still conceals anything behind it, even if you know it’s not real. Mirages are very poor at tactile sensations and heat; they can create the impression of them, but cannot generate scorching heat or the violent blow of a sword. This type of illusion is known to mages, with the simplest being Illusionary Mural and Audible Clamor.
The second type, variously called fictions, hallucinations, and phantasms, target a single individual with the perception of their senses being stimulated. Little is actually created; outsiders watching someone fighting a phantasmal warrior may see shapes and shadows engaging their ally, but will more likely see nothing at all, even as the target reels backwards from a mighty blow or curls up under a blast of phantasmal flames from a fictional fiery foe. Fictions can kill, but seldom cause wounds; a successful saving throw against them frequently dismisses the effect entirely. A simple mage spell that makes use of fictions is Phantom Irritation; no one else will hear the buzzing or feel the tickle, but it will be enough to distract the target.
Why study illusion magic? The magic of a regular mage is very powerful, and has real effects. But it is also relatively inflexible. If you do not know the spell, you cannot do the effect. A mage who does not know Illusionary Mural will not be able to create an illusionary mural. If they know it but have not prepared it, they will be unable to use it to its full effect. Illusionists, by contrast, have studied the fine art of illusioncrafting, and are able to create the mirages and fictions they wish, at the expense of the higher secrets of mage magic.