1

Between two knit stitches: Bring yarn to the front between the needles, take it over the top of the right-hand needle and then knit the next stitch.
This is sometimes known as a yarn forward (yf or yfwd)

2

Between two purl stitches: Take the yarn over the right hand needle, then under between the needles so that it's at the front of your work again. Purl the next stitch.
This is sometimes known as a "yarn round needle" or yrn.

3

Between a knit stitch and a purl stitch: Start with yarn at the back. Bring yarn forward between the needles, take from front to back over the top of the right-hand needle, and back through the needles again to the front ready to work a purl stitch.
This is sometimes known as "yarn forward round needle" or yfrn.

4

Between a purl and a knit stitch: Take yarn to the back between the needles. Bring the yarn over the top of the right hand needle and to the back again between the needles.
This is sometimes known as "yarn over needle" or yon.