Slip, Slip, Purl (ssp)
The ssp, or slip-slip-purl, is a left-leaning single decrease. It takes two stitches and turns them into a single stitch that slants toward the left on the right side of the work. It is generally worked on the wrong side and looks like a slip, slip, knit (ssk) on the front.
To work a ssp, slip the first stitch on the left-hand needle as if to knit, then slip the second stitch on the left-hand needle as if to knit. Slip these two stitches back onto the left-hand needle, preserving the twist you've just added. Finally, purl these two stitches together through the back loop.
The following video demonstrates how to work a ssp English style:
And this video shows Continental: