Effects: Particle Density
There are five options for the Particle Density setting. According to the tooltip (and common sense), this option "controls the number of particles used in effects caused by spells, fires, etc. Decrease to improve performance."
Despite the granularity Blizzard gives us here, the two extremes (Ultra and Low) are the most telling. With that said, I haven't found particle density to be critical in playing World of Warcraft. If you have to dial this one down from Ultra in the interest of improving performance, you won't be sacrificing much in the way of gaming experience.