Dictionary Avatar Art: This is when an artist will take a users avatar and draw a picture of it. Your avatar is the chibi-like character whom you customize with various items.
OC Art: Stands for "original character." This is when a buyer gives you specific descriptions of what they would like you to draw. They sometimes even provide an example someone has already created of their character(s). OC art typically costs more than avatar art.
Gray-scale: Gray-scale art is when an artist draws the image with a pencil, scans it, and than sends it to the user. Sometime they may use a tablet to create the art, or do minimal editing to the scan to improve the quality. A tablet is a device used to draw art directly onto your computer.
CG: CG stands for Computer Generated. This is when the artist takes their initial gray-scale sketch and colors it in, shades, or adds special effects in a program such as Photoshop.
Anime: If you do not know what Anime is, feel free to Google it. It is a style of art (or cartoon if you will) that is typically associated with Japanese culture.
Chibi: Chibi is a specific style of anime, where the proportions of the body are changed. The result ends up looking somewhat "cute." Feel free to Google this as well.
Realistic: Realistic art is when the artist draws the image with realistic features, proportions, etc.