To decrease a stitch simply means "subtracting" the stitch (or a few of them) from the number of stitches in the row for shaping and design. Different techniques of decreasing allow you either to make decreases hardly visible or, on the contrary, make them clearly visible in the knitted fabrics. This way they will create additional decorative effect to your design.
There are dozens of ways to make increases which designers use for the decorative purposes. Videos presented on this page show you only two popular methods of decreasing stitches:
1. To work two stitches together through the front loop ( K2tog OR p2tog).
2. To work two stitches together through the back loop. (ssk OR ssp)
You can make decreasing either in the knit row ( K2tog = knit 2 stitches together) or in the purl row (p2tog = purl 2 stitches together ).
I would like to remind you , that all knitting stitches and technique are EXACTLY the same for both CONTINENTAL and English style of knitting. If you are not a very experienced knitter and some differences between two styles of knitting are confusing for you, here is very detail help at KnittingHelp. Since a vast majority of people is the visual learners let's watch the videos.
It can be done either in the knit row: ssk= slip, slip, knit 2 stitches together. Decreasing a stitch in the purl row has abbreviation ssp= slip, slip, purl 2 together.
SSK will produce left-slanting decrease in the knit row. SSP will look as a right-slanting decrease in the purl row.