What are the differences between oblique sketches and isometric sketches?

In an oblique sketch you have a horizontal x-axis and the z-axis comes off of it to show depth. In an isometric sketch the x-axis and the z-axis are at angles, and the angles of all three axis are 40 degrees each, or 120 degrees total.

Oblique Sketch: http://www.me.utexas.edu/~rbarr/images/Oblq-2.gif

Isometric Sketch: http://ppdl.engr.ou.edu/ame2303/step5i.jpg

as you can see, in the oblique sketch the axis is perfectly horizontal while the isometrix axis is not.