Decreases remove stitches, narrowing the knitting. The most common ones you will find in patterns are K2tog (knit two together) and SSK (slip, slip, knit). They're easy, just watch the videos. There are also double decreases, which remove two stitches at once. I don't feature any of these on that decrease page, but you'll find at least one in the Glossary (see, for example, CDD).
The sampler below contains both increases and decreases. Since it was knit from bottom to top, the lower half of the diamonds contain increases, and the top half contain the decreases. All of these decreases are worked on the knit rows only, with the exception of SSP. I have named KRPR, after the actions required to make it.
k2tog
(Knit Two Together) The most
common right-slanting decrease A perfect, neat decrease that slopes
to the right. It's so neat and tight, it is tighter looking
than any left slanting decrease.
...Knit two stitches together as if they were a single stitch.
SSK
(Slip Slip Knit) The most common left-slanting
decrease A very commonly used left slanting
decrease .
...Slip one stitch, then slip the next. Insert left needle
into the front loops of the slipped stitches and knit them
together from this position (through the back loops).
k2tog
SSK "Improved" Lays slightly flatter than SSK. Some
prefer it as an even closer match to k2tog. ...I can't make
up my mind, I like them both!
...Slip one stitch as if to knit, slip the next stitch as
if to purl, knit the two together through the back
loops.
KRPR
(Knit, Return, Pass, Return)
Looser than k2tog, but in this way matches the left slanting
decreases even better. ...Knit 1, return knit stitch to left needle, pass
the next stitch over the knit stitch, replace to right needle
by slipping purl-wise.
SKP
aka sl1, k1, psso
(Slip Knit Pass) or (Slip One, Knit One, Pass the Slipped Stitch
Over) This is the same resulting stitch as SSK. Use them interchangeably, whichever method you prefer.
...Slip one stitch, knit the next, pass the slipped stitch over
the knit one.
Crossed
Right
Slanting Decrease (no video at present)
This decrease twists the stitches around to match k2tog tbl. ...Slip 1 k-wise, slip another k-wise, return them
to left needle, and knit them together (through front loops)
k2tog tbl (Knit
Two Together Through the Back Loops) This twists the stitches around and
consequently tends to zig zag like crazy. It's not a great
match for k2tog, clearly, but can be used decoratively.
...Knit two stitches together through the back loops.
k2tog
SSP (Slip Slip Purl) This is actually the best looking match
for k2tog. Nice and tight and neat looking!
Unfortunately it needs to be done on the reverse
side (the purl row), which usually means a row later or earlier
than when it is called for. If you're
knitting in the round, you can't use this decrease, since
the purl-side never faces you.
...(On the purl row) Slip 1 k-wise, slip another k-wise. Return
slipped sts to left needle. p2tog tbl: Insert right needle
up into back loops of the two stitches and purl them together
from this position.
k2tog
SSP
k2tog
SSP
"Yarn
Over Decrease
Right-Slant" A right-slanting decrease to match
the Yarn Over increase below it.
...k2tog, yarn
over, k2tog
"Yarn Over Decrease
Left-Slant" Here is a decrease to match the Yarn
Over increase below it. YO is included to create the hole
in the knitting. But since yarn over is an increase, it is banked by
two left slanting decreases to equal 1 total decreased stitch.
...ssk, yarn over, ssk