I've had a spot of bother with trying to knit moss stitch and it turning into a rib stitch, and vice versa, so now I've finally got my head around it (I think!) I thought I'd put a post up explaining how to not make that mistake for reference, and in case it's useful to anyone else.
Moss and rib stitch use the same knitting pattern, that is, knit 1, purl 1. The difference is in how the rows of knitting relate to each other. So, the Golden Rule is:
To do rib stitch, you must make sure that knit stitches always lie on top of knit stitches, and purl stitches lie on top of purl stitches. So, you have to look at the row you have just knitted (with knit 1, purl 1) to see whether the first stitch you are going to knit on the next line will fall on a knit or a purl on the line below. If it will fall on a knit, start with a knit; if it is a purl, start with a purl.
A purl stitch looks like a knot, and a knit stitch looks like a V.
To do moss stitch, you must do the opposite, that is, ensure that a knit stitch always falls on a purl stitch, and a purl stitch falls on a knit stitch. For moss stitch to work you also need an even number of stitches. I have no idea why!
I may add to this post if any other tips surface regarding rib and moss stitch! :)