Introduction to UV Mapping - Part 2 Terminology

A point in space is defined by the set of coordinates x,y,z.  
x is left to right horizontal, y is up to down vertical, and z is foreground to background depth.
In Maya, x is red, y is green and z is blue.
Anytime we are talking about XYZ space, we are referring to 3D space.

A point in 3D or XYZ space

"UV" mapping refers to a separate, related set of coordinates where
 U is left to right horizontal and V is up to down vertical.
Whenever we talk about "UV's" and UV space we are always talking about flat, 2D space.

Look Ma, no blue! The humdrum 2D world of UV space as seen in the UV editor

NerdNote: You'll also hear UV space referred to as UVW space. Yes, there is a "W" coordinate equivalent to 3D "Z". For our introductory purposes, however, we should consider W as an unholy abomination against nature not to be spoken of in polite society.