Unless your instructions specifically tell you to slip a stitch knitwise, always slip a stitch as if you were going to purl it. Here’s a look at stitches being slipped both purlwise (a.) and knitwise (b.).
You frequently run across slipped stitches in methods for decreasing stitches — when you want to reduce the number of stitches you have on your needle. They also form the basis of a family of stitch patterns. Like garter stitch, slip-stitch patterns are stable and lie flat — and they’re a breeze to knit.