In Knitting, there are 2 main decreases - ones that lean to the left and ones that lean to the right. We will be only looking at 2 here:

Knit Two Stitches Together (k2tog):

This decrease leans to the right.

Insert needle as to knit through the next two sts on the needle, knit them as one stitch.

Slip, Slip, Knit (ssk):

Insert needle into next stitch as if to knit it, slip it onto the other needle without knitting it,

do the same to the next stitch,

insert your needle through the back of the two sts just slipped and knit them together as one stitch.