Well pallette knives are generally used with oils because of the fact that oils are very dry paints are are much harder to mix with a brush.
Also, with pallette knives, you can get some really nice effects when you apply the paint to the canvas.
Bob Ross is usually a really easy example to show for this, but it seems like all his videos have been pulled from the public domain... All i can find now is weird spoofs...
Anyway, i found this one on stage 6 that shows him using a pallette knife. He was famous for his pallette knife mountains and unfortunately this one doesnt show that, but it should give you some idea...
http://www.stage6.com/user/glacier626/v ... amp;-Woods
The pallette knife techniques comes into play at about 13:00 minutes in. If you've never seen Bob Ross before you are in for a real treat too!! ;)