A slip stitch is a "zero height" stitch used in standard crochet. It is often used in patterns to create an almost invisible join to another part of the garment. It is also used to fasten sets of stitches back to the main fabric. The slip stitch is used in such a manner due to the fact when it is applied, it lies very close to the previous row, creating very little height difference from row to row.
The most common place this stitch is seen is when joining crochet in the round to create a tube (as opposed to a continuously crocheted spiral, which is not joined).
When used to create a fabric, the fabric has a very dense, cushony texture, and becomes very stretchy in all directions. In general, slip stitch fabric is created with a hook of matching size or 2 to 4 times larger, as compared to the size of the yarn.
The slip stitch can also be used with great effect to mimic the knitting knit stitch, or the tunisian crochet knit stitch.
Styles in which slip stitch is the main stitch are called Bosnian crochet or slip stitch crochet. The terms are synonymous.