In SSK you:
Slip 1 as if to knit.
Slip another as if to knit.
Then your left needle goes into the FRONT of the slipped stitches from left to right. Your R needle is behind the left needle and you work the stitch.
In K2tog there is no slipping.
Your right needle goes into the left most stitch you want to knit together, then continues through to the right most stitch. You basically are knitting like you normally do, but you go through two stitches at once instead of one stitch.
Check out the decrease videos (the two you asked about are at the top): DECREASE VIDEOS