Yep, Lara, you gave me the idea for the previous post. What's a yarn over (YO in pattern speak)?
A yarn over is when you wrap the yarn around the needle before making the next stitch. Visualize looking at your needles. You're in the middle of a knit row (works with purl too). Before you knit the next stitch, you wrap the yarn from back to front around the needle, insert the needle in the next loop and make your stitch. This adds a stitch to the row. (Usually YOs are combined with decreases to maintain an even stitch count. I'll talk about those another time.) When you come back with the next row and purl or knit the yarn over you end up with a hole in your work because this is a loose stitch. You've made lace!
Sometimes beginning knitters make yarn overs unintentionally when they forget to move the yarn to the back after finishing purl stitches and starting knits again. This is why sometimes you suddenly have more stitches than you started with.
As far as beginning and ending stitches on the purl side of a piece (this assumes you're knitting flat stockinette). I usually just purl them but this can make the edges curl in. No biggie if you're planning to seam pieces together, but not so pretty on a scarf. If you knit the first and last stitches of a purl row, you make a very narrow garter stitch edge. This doesn't curl up as easily. Another possible method is to slip the first stitch of the purl row as if to purl (transfer it from the left needle to the right without purling) then purl the row. At the beginning of the next knit row, slip the first stitch as if to purl (keep yarn in back though!) as well then knit the row. I've never tried this on a scarf or blanket, but this is the way to put the edge on the heel flap of a sock, more or less!