Most knitters know that "decrease" meant to reduce the number of stitches on your needle.  There are many ways to do this, and the method you use depends on the way the piece is shaped and the look you are trying to achieve.  Decreases can be worked as knits on the knit side of your work, or as purls on the purl side of your work.  You will want to consider the slant of the decrease when you choose the one to use on a particular edge as you generally achieve a more professional look if the decrease slants in the direction of the shaping.  The explanations below will help you choose among the available options.                                                              

K2tog/P2tog -- Knit 2 together/Purl 2 together -- Single decrease, slants right

SSK/SSP/SKP -- Slip, Slip, Knit/Slip, Slip, Purl/Slip, Knit, Pass slipped stitch over -- Single decrease, slants left                          

K2togtbl or P2togtbl -- Knit 2 together thru back loop/Purl 2 together thru back loop -- Single decrease, slants left

K3tog/P3tog  -- Knit 3 together/Purl 3 together -- Double decrease, slants right

SK2P -- Slip one, knit 2 together, pass slipped stitch over -- Double decrease, slants left

S2KP --Slip two, knit one, pass slipped stitches over -- Centered double decrease