I've seen both terms used for the same thing, but I've also seen them used differently. To me, YO (Yarn Over)has always meant bringing the yarn completely over the needle, from back to front and to the back again, creating a new stitch.
YF (Yarn Forward), on the other hand, has always meant simply moving your yarn from the back to the front, just like you do when you go from a knit stitch to a purl stitch. As Suzeeq, if you do a YF and then knit a stitch, you end up with a YO, so it can be confusing. I guess it all depends on how the patterns uses it!
Hope I didn't confuse you even more!