A yarn over is an increase .... you are looping the yarn around the needle to create an extra stitch. The yarn goes all the way around the needle and back to the position it was in (for example, back to the knit position).
A yarn forward is NOT an increase, you are just bringing the yarn from the back of the work (in the position it would be to knit) to the front of the work (to the position it would be to purl). You are just bringing the yarn to the front by moving it between the tips of your needles ... the yarn doesn't go over the needle and make an extra loop.
There is a video on how to do a yarn over on the videos page in case you want to see it demonstrated.
Hope this helps clarify! Good luck!