Gradient banding is the effect in which the transition lines in a gradient are too visible. For Photoshop, this is more often an issue for onscreen images than it is in print (it’s rather the opposite in Illustrator)
on KelbyTV (one of my favorite places) Dave Cross has a very interesting tutorial on some anti-banding techniques.
You can watch the video here:
http://kelbytv.com/askdave/2011/07/11/reduce-banding-in-gradients/