The back of a knit stitch is a purl, the back of a purl is a knit. If you knit every row or purl every row you're going to get garter stitch where the purl bumps look more prominent. If you stretch that lengthwise, then you can see the knit sts better. When you alternate them - knit a row, purl a row - then you have stockinette stitch and all the knits are on one side and all the purl bumps on the other side.
sue- knitting heretic