Elder Key of Strange and Mysterious Knitting Abbreviations

Decreases
There's a lot of ways to make stitches disappear.

K2tog: Knit two stitches together.
Ssk: Slip, slip, knit; slip one stitch knitwise, then one stitch purlwise, and knit these two stitches together through their back loops.
P2tog: Purl two stitches together.
Ssp: Slip, slip, purl; slip one stitch purlwise, then one stitch knitwise, and purl these two stitches together through their back loops.
Cdd: Centered double decrease; slip two stitches as if to knit together, knit one stitch through the back loop, pass the slipped stitch over.
Sk2p: Left leaning double decrease; slip one stitch, knit two stitches together, pass the slipped stitch over.

Increases
The stitches are coming from inside the knitting!

K1f&b: Knit into the front, then the back, of one stitch.
R-inc: Right lifted increase; lift the stitch one row below stitch on left needle and knit through the front leg.
L-inc: Left lifted increase; lift the stitch 2 rows below the stitch on right needle and knit into the back of it.
Yo: Yarn over needle, or yarn around needle.

Assorted Weirdness
You want me to do what now?

S1k, Slp: Slip one stitch as if to knit, or as if to purl, depending.
Psso: Pass the slipped stitch(es) over.