G90 is a simple "box" turning cycle that can be either straight or tapered (with I). Let's say you have a 1.5" bar you want to turn down to 1.375 x 2.0 long. X and Z are the "target" points. U and W can be used in place of X and Z to specify an incremental distance to the target, but IMHO it makes more sense to use X and Z. I is the radius difference when commanding a taper. The machine will rapid to X, feed to Z, feed back to the starting X, and rapid back to the starting Z.
G00 X1.6 Z0.1 M08 (RAPID TO CLEAR)
G90 X1.405 Z-2.0 F0.012 (ROUGH PASS)
X1.375 F0.006 (FINISH PASS)
G00 X3.0 Z2.0 M09
The same cut with a taper from 1.312 to 1.375 would include an I (radius change amount):
G90 X1.405 Z-2.0 I-0.0315