The purls have bumps, the knits are smooth. Each stitch has two sides, on one side it's a knit and on the other side it's a purl. So you do the stitches as they face you..which means when you are holding the knitting in your left and are ready to start knitting again. Here's a sample that shows what they look like. It's ribbing, but it'll be same with seed stitch... just closer together.