I’m a beginner & trying to learn how to knit. I know how to do the knit stitch (at least I think I do) and went to learn how to do the purl stitch. Is the main difference between knitting and purling is that when you knit a stitch the yarn is behind you & when you purl a stitch the yarn is in front of you?
Yes, the yarn position is different for the purl stitch than it is for the knit stitch … but there is also a difference in how you insert your needle into the stitch being worked. Check out the video page on this site … there are great videos on how to do both stitches, and usually seeing visually how it is done is the best way to figure out how to do it properly. 
There are two main differences - where the yarn is and how you insert the needle into the stitch. For the knit st, you poke the R needle from the front to the back (where the yarn is so you can wrap it around) and on a purl st you poke the needle in from the back to the front (where the yarn is so you can wrap it). Do check out the videos as well.