A yarnover, or YO, is pretty much an American term. The British term, you may have been told, is "yfwd", or "yarn forward". Right? Well, sort of.
British knitters use yfwd, but they also use three other terms: yrn, yon, and yfrn, standing for "yarn round needle", "yarn over needle", and "yarn forward round needle", respectively.
Shove your bugged-out eyes back where they belong, and calm down; it's not that hard. The four terms basically differentiate between the slightly different steps you do depending on whether your preceding and following stitches are purls or knits, and each of them stands for one of the four possible combinations of that.
Since we broke the YO down in the last post to three steps, two of which may or may not be done, let's refer back to those: step 1 is where you move the yarn forward in preparation for the YO if it's not already there; step 2 is the YO itself, where you flip the yarn over the needle; and step 3 is where you move the yarn forward again if the next stitch needs it to be there. We'll see how each of the Brit terms indicates a different combination of those.
YFWD: This is a YO done between two knit stitches. This is the type you're doing if you follow those bad instructions I referred to in the last post: you bring the yarn forward between the needles, and then when you work the next stitch, since it's a knit, that automatically causes you to bring it over the top of the needle. It's the same as doing steps 1 and 2 of our YO. And if this were the only type of yarnover maneuver that existed, those instructions wouldn't be bad; unfortunately for them, it isn't.
YRN: This is a YO done between two purl stitches. You're starting with the yarn in the front, so you bring it all the way around the needle until it's back in the front again; you're doing steps 2 and 3 of the YO.
YON: This is a YO done after a purl and before a knit. You're starting with the yarn in front, and you want to end with it in back, so you just flip it over the needle; it's step 2 all by itself.
YFRN: This is a YO done after a knit and before a purl. The yarn is brought forward and then brought all the way around the needle until it is forward again; it's steps 1, 2, and 3 of our YO method.
You can see from this how the various names go with the steps that you do, and how they fit with YOs by our method.
Every once in a while, you may encounter a pattern that uses YFWD in a place where it clearly is meant to indicate some type of yarnover, but it's not between two knit stitches. Usually, what this means is that it's an American designer writing for a British publisher, and someone has bulk-substituted YFWD every time the designer wrote YO, without thinking about whether that's the proper type to do in that circumstance. Occasionally, however, this may mean that they want you to bring the yarn forward for some other reason, such as to slip stitches with the yarn in front, so double-check that you understand what you're being asked to do, and then go ahead and do the YO steps that are appropriate if that's what's called for.