Tridimensional shapes and routes are described using three coordinates X, Y and Z. Cutting down simple the X-axis is left to right, the Y-axis down to up and the Z-axis is back to front. The Z-axis requires a further abstraction if you want to comprehend it in a theoretical basis, but talking practical it's just the stretching of the image.

The unit for describing the distances in your virtual world is relative, but almost everyone uses the meter unit. Of course if your making a model of a molecule you're not going to use meters but rather Angstrom.

Angles' unit is radians, this confuses some but after a while you'll get around it easily. Just remember that pi = 180º, which is approximately 3,1416 rad, so pick up your calculator and divide, nothing more to it.