Like memo and Jimbobwe said, they're motion trails.
There are two types of trails, the color based ones and texture trails. The solid color trails include items like Aqua Trail, Acid Trail, etc... These are normally inexpensive compared to texture trails.
Texture trails are the same thing, but you upload an image that basically repeats itself to create the trail. NOTE that you don't just find someone that draws the trails and buy the image... You also need the Texture Trail Item to be able to upload it onto your character.