Alright, knitters. Now that all of your crocheting friends are cranking out ripple blankets and lacy scarves thanks to last week's blog post, it's your turn. The good news is, your choices in decreases are pretty straight forward: k2tog, ssk (or skp) will turn two stitches into one, and for multiple decreases, you'll either see sk2p or k3tog (or 4, or 5, etc.). Mostly, they're worked exactly how you think they would be, so instead of covering the mechanics I'm going to just give you a link to our Learn to Knit tutorial for each stitch and then talk about when it's appropriate to use the different decreases.
K2tog ("knit two stitches together") and ssk ("slip, slip, knit two slipped stitches together") are both simple decreases: you're turning two stitches into one. The magic of these stitches is that they lean in different directions: a k2tog leans to the right, while an ssk leans to the left. Ever notice on a raglan sweater how the stitches along the seams seem to point to each other? That's because they are what is called "paired decreases": one left -leaning decrease and then one right-leaning decrease. You'll see this in lace a lot, too. You will also come across variations on these decreases, such as working them through the back loop (k2togtbl and ssk tbl) or purling rather than knitting the stitches (p2tog and ssp).
A couple of notes on the ssk: in some patterns, especially older ones, you'll see an skp ("slip one, knit one, pass slipped stitch over" called for instead of an ssk. These two decreases are interchangeable, but many people find that an ssk lies flatter and looks more like a reversed skp, which is why most modern patterns will call for the ssk. Also, when you are decreasing in knit (i.e., ssk as opposed to ssp), you slip as if to knit, leaving your yarn at the back of the work. (A quick tutorial on slipping stitches can be found here.)
For multiple decreases, the most common double decrease is the sk2p, which stands for "slip one, knit 2 together, pass slipped stitch over [the stitch you just created with the k2tog]". You'll also see k3tog pretty frequently, and for decreases of more than two stitches, you'll usually see kXtog, where "X" is the number of stitches. Note that the number of stitches you're decreasing is always one less than the number of stitches you are working. So a k5tog decreases four stitches, a k3tog decreases two stitches, etc.
Sometimes you will also see a bind-off used to decrease stitches. This is most commonly done at an armhole or neck edge, and the stitches are bound off as normal.
That's really all there is to it. Now you're all ready to knit a nice lacy wrap to ward off those chilly little spring breezes.