The direction in which the yarn is wrapped around the needle affects the size of hole created by a yarn over. If you are working on a project that has lots of yarn overs between different types of stitches you may want to swatch and alter the yarn over directions so that your yarn overs are of a uniform size. Ishbel is one of the patterns where this question comes up regarding the size of the yarn overs not matching on the edges and either side of the spine stitch.
Yarn over — bring the yarn to the front between the two needles and then to the back over the right needle. If the next stitch is a purl bring it to the front between the needles.
Backwards yarn over — bring the yarn to the front over the top of the right needle. If the next stitch is a knit bring it to the back between the needles.
Backwards yarn overs mount the created stitch on the needles so that on the following row you must work into the back leg of the stitch to avoid twisting it.
Advantage of combining yos
As can be seen in the above swatch regular yarn overs worked between a knit and a purl create a substantially larger hole than they do when the purl stitch comes before the yarn over and the knit stitch comes after. Working a backwards yarn over between the knit and purl can give a more symmetrical result.
On a shawl like Ishbel all of the yarn overs at the edges on the right side are worked between two knit stitches. But on the wrong side the centre stockinette portion is purled and the garter stitch edges are knit. Consequently at the beginning of the row you work k3, yo, purl and at the end you work purl, yo, k3. Prevent the yarn overs at one side from being larger by working that first yarn over before a purl backwards. Don't forget to knit into the back leg on the next row!
Other yarn overs
Double yarn over — yo2
The yarn is wrapped around the needle twice and makes a larger hole than a single yarn over. On the next row the double yarn over is often worked into twice (k1, p1) or (p1, k1) creating two stitches. The double yarn over is used in Sherilyn for the edging and in wee Liesl to create the buttonholes. Always treat a double yarn over as a single loop on the subsequent row.
Elongated stitch multiple yarn over
Some elongated stitches are worked by wrapping the yarn several times and dropping the extra loops on the following row.
Backwards yarn over — used in Jeny's surprisingly stretchy bind off
Twisted yarn overs — used as an alternative to m1
Steeks! We usually see steeks cut through a column of extra stitches but multiple yarn overs can also be used: