Slip stitches are a great way to create texture, pattern and add interest to a fabric. It is also commonly used when working decreases. To slip a stitch you simply move it over from the left hand needle to the right hand needle without knitting or purling through it. Techniques and patterns will usually specify whether you are slipping knitwise or purlwise.
When you slip a stitch knitwise you insert the right hand needle as if to knit, which results with the front loop in the back and the back loop in the front, twisting the stitch. When you slip a stitch purlwise you insert the right hand needle as if to purl. The stitch remains with the front loop at the front and back loop at the back.