For those who aren't well-versed in higher level math, the z-axis is a third dimension. It's more accurate to say that the z axis extend out of the paper toward and away from you, not diagonally.
The x-axis represents detail. Those who score low in detail tend to have simpler designs. They may only use a few different blocks for the build. Overall shape might be a cube or your typical angled-roof house. Normally newcomers to Minecraft are low with detail, although seasoned players might still fall under this category.
A high score in detail means that every square meter of a build is put to good use. Overall shape and the small things are carefully thought out and combined in a tasteful way.
The y-axis represents size. A low score in size means you tend to stick to making smaller things. Houses, farms, shops, and stables tend to be smaller in size. A high score in size means you tend to build big. Examples would include skyscrapers, mansions, mob traps, and other things which are, well, huge.
The z-axis represents functionality. I have noticed that many people tend to lean toward making things which are purely for aesthetics, and others who are more into functional builds. A low rating in functionality means the build doesn't do much except look good. High ratings in functionality means the build does something. Redstone is an obvious example of functionality, but farms, furnaces, storage, and mob traps are also functional.
Examples:
Let's do these measurements on a scale from -10 to 10. Those numbers are arbitrary but they do the job.
Me: If I were to evaluate myself, I'd say I was a low-mid (-5) on detail. I'd also go low-mid (-3) for size, as I don't do very well making big things, but I also don't like stuff that is too small. As for functionality I rate myself high (8) because the things I make usually serve a purpose.
EthosLab: I don't think he's atrocious at detail, but also not the best. A medium rating (0). He's like me, in that things that are big really take energy to stick to, but smaller stuff is fun, so an upper-medium (4). He practically maxes out the functionality scale (10).
VintageBeef: Good with detail, but not superb (7). Definitely big-scale builder (7). Functionality is less of a priority for him (5).
Arkas: Very impressive at detail (8) and size (10). Not too bad at functionality (6). An all-around builder.
BdoubleO100: Superb with detail (10) and size (7), although he routinely does big and small. More of an aesthetics person than function, but he can do it too if needed (3).
