Generally, they're the same. A yo is bringing the yarn forward to make an increase when the next stitch is a knit. A yo before a purl requires you to bring the yarn over the needle and back to the front; between a knit and a purl, it goes over the needle, too.
Sometimes yarn forward means just to bring it forward for a slip stitch pattern, for example. Since there's a yarn forward, there's also a yarn back.
If a pattern says yf, k1, then it's essentially a yo, so it really depends.
I used to think they were exactly the same, but there are differences depending on the pattern.