Illusion spells deceive the senses or minds of others. They cause people to see things that are not there, not see things that are there, hear phantom noises, or remember things that never happened. Illusions come in five types: figments, glamers, patterns, phantasms, and shadows.
For a list of all spells which are of the school of Illusion, see Category:Illusion spells.
There are a few minor save-or-die spells like Weird and Phantasmal Killer, although they both have both a will and fortitude save, making them much easier to resist except for those classes that only have a high reflex save.
For utility, there are also the Shades, Shadow Conjuration, and Greater Shadow Conjuration spells, which provide access to additional spells. If applicable, these are cast at a higher save DC due the use of a higher innate level. However, they do not benefit from Spell Focus (Illusion), and are considered to belong to the same school as the original spell.
These spells are useful to sorcerers who need to save on the amount of spells they can cast, or a wizard who wants to take advantage of the choice of spells, although they are not nearly as versatile as in 3.5.
Because of the distinct lack of illusions, there is simply no need for disbelief saves. The spells which use saves similar to disbelief (Weird and the like) already have the saves noted in the 3.5e spell descriptions anyway.
The spells Shades, Shadow Conjuration and its greater version should all be able to pick any spell from X and lower at the time of casting. Due to the real time combat, it likely this is the reason they were streamlined or made into only 5 choices.