Single Decreases
(These "lean" visually to the right or left, depending on how you work them).
k2tog (Right leaning). Knit 2 together. On WS, p2tog: Purl 2 together.
k2tog-b (Left leaning). Knit 2 together through back loop. On WS, p2tog-b: Purl 2 together through back loop, inserting needle from the left.
ssk (Left leaning). Slip, slip, knit. Slip 2 stitches, one at a time, as if to knit. Put left needle through front of both these stitches and knit together. Sl-k1-psso: (Left leaning). Slip 1 stitch, knit 1 stitch. Pass slipped stitch over knit stitch.
Double Decreases
(These are both called "central" double decreaseses, although I think that's a bit misleading. In the first version, sl1-k2-psso, the topmost stitch leans to the left. In the second version, the top most stitch is straight. The first makes more of a texture, while the second makes a ridge.)
sl1-k2tog-psso Slip 1 stitch, knit 2 together, pass slipped stitch over.
sl2-k1-p2sso Slip 2 stitches as if to knit 2 together, pass 2 slipped stitches over.
Increases
yo Yarn over. Bring yarn between needles to front and then back over the right hand needle. If the next stitch is a knit, your yarn is in place. If the next stitch is a purl, bring the yarn to the front between the needles. The strand created across the needle becomes a new stitch.
k1, yo, k1 Knit 1, yarn over, knit 1 into one stitch. on WS p1, yo, p1.