You can do SSK as Pious Poet describes, it creates the same results, but the term comes from "Slip, Slip, Knit." Where you slip 1 k-wise, slip another k-wise, then insert the left needles into the front loops of the slipped sts and knit them together from this position.
And SSP is "Slip Slip Purl." Slip 1 k-wise, slip another k-wise, transfer slipped sts. to left needle, and purl together tbl. (Not the same decrease Pious Poet mentioned. That one is SPP--slip, purl, pass.) A video of SSP is coming soon, on the decrease page. I just learned about this decrease, and it's my new favorite decrease! It is done on the purl row, not the knit row, but on the knit side, it creates a very neat left slanting decrease that is the best match to k2tog I've seen! It's the only left slanting decrease that doesn't stretch out the stitch, and look bigger than k2tog.
Pious Poet, we were just talking about the "which way to slip" question a couple weeks ago on another thread. (It might have been on another forum? I think it was here. Anyway...) Everyone had a different answer as to which way you were supposed to slip if the directions didn't specify. We were all wrong, including me! I finally looked it up in Vogue Knitting, and the answer I gave you came from them. So far it seems to be an unfailing rule! But certainly not an obvious one!