The slip stitch is useful when a crochet stitch is needed that doesn't add any height.
How To Slip Stitch: Insert the crochet hook in the next stitch to be worked, wrap the yarn or thread over the hook, pull the yarn or thread through both the stitch to be worked and the loop already on the hook, all at the same time.
Uses For Slip Stitches
The slip stitch can be used to join the last stitch made to another part of a row or round.
Slip stitches can also be used as a way to join afghan squares or other crocheted pieces together, as an alternative to sewing the pieces together. Photo Example
The slip stitch is sometimes used to move the hook over to another part of a row or round, when needing to skip some stitches as part of a design, without adding any height to the part being skipped.
Another use is as a decorative stitch on an edge, to give a finished look to the edge of a crocheted item.
Article copyright (c) 2007 by Sandi Marshall, licensed to About.com, Inc. Article written from my own crochet experience, with knowledge gained from many years of avid crocheting.