We've covered various methods for doing a true cable, so let's turn our attention to mock cables. What are mock cables? They're not a single thing, but a group of various techniques which produce a fabric that looks as though it's been cabled, but don't actually involve moving stitches around, which is the hallmark of a true cable.
One of the most common types of mock cable is the twist stitch. A twist stitch is done on two stitches, by pairing a decrease across both stitches with working one of the stitches a second time; this gives you a slanted stitch which appears to move across the ground on which it lies, mimicking the true twist where one or more knit stitches move over one or more purl stitches. The twist stitch is more prominent than the true twist, and is often used on a stockinette ground, where the true twist is nearly always used on reverse stockinette.
The right twist (RT) is usually done by knitting two stitches together, but leaving both of the old stitches on the left needle, and then knitting the first stitch again by itself, before letting the old stitches go. This gives you a prominent stitch slanting toward the right, with a following stitch almost tucked beneath it. There is a variation on this wherein one simply knits the second stitch by itself without moving the first stitch, and then knits the first and takes both off the needle; this variant is closer to a true cable, but it pops less dramatically than the version with the k2tog, and is no easier to execute.
The version of the left twist (LT) most commonly seen is to knit into the back of the second stitch on the left needle, and then to knit the two stitches together through their back loops. However, I find this not to be a marvelous match for the RT, just as k2tog-tbl is not a great match for k2tog. If matching is desired, as in a pattern which includes both twists, I prefer to vary the LT by first slipping each stitch individually knitwise and then replacing them on the left needle, before knitting the second through the back loop and then both together through the back loops. This matches RT in the same way that SSK matches k2tog. It's a bit fiddlier, but definitely worth the trouble. As with RT, there is a variant that is simply to knit the stitches out of order -- in this case, the second stitch is knit through the back loop, and then the first is knit through the front, and both old stitches are removed together. Also as with the RT, this pops less dramatically than the version which includes the decrease.
Hello Yarn's cable-twist socks present an interesting variation on the LT; in this variation, the second stitch is knit through the back, and the first is merely slipped. Because the slipped stitch is elongated, it is prominent in a way it would not be if it were merely knitted, and this is a nice dramatic mock cable, quite easy to execute. Unfortunately, a well-matched RT version is not readily available, because slipping the second stitch first isn't really feasible. Edit: For an interesting variation on the cable-twist socks, check out Christine Selleck's clog socks modification, with the mock cables continuing down the heel.
There are many other types of mock cables. One common method is to use biasing -- increases and decreases on either side of several columns of stitches are used to make those columns weave in a serpentine pattern. A great example of this is Sockbug's River Rapids socks; you can see them knitted up in a solid, which I think shows the patterning better, at Krys' blog here -- see how the YOs on the right side push the columns to the left, and then YOs on the left push the columns to the right? This is a great technique and makes a fun knit.
Another common technique for mock cables is to have a slipped stitch or a yarnover which is passed over multiple stitches, so that what would otherwise be a rib is cinched together. This type of mock cable mimics the drawing in at the crossing point of true cables, whereas the biased type of mock cable concentrates more on the movement of the columns to mimic the appearance.
Why not just do true cables? There are a number of reasons. To a degree, the use of mock cables might be for simplicity and speed of knitting, as true cables are generally somewhat slow to knit, but there are other advantages which can be obtained. Cables pull in a great deal, and often use substantially more yarn than an equivalent amount of stockinette; mock cables mitigate these effects somewhat, so they can produce a more flexible and more economical fabric. Another reason, especially for the twist stitch type of mock cable, is that these stitches pop off a stockinette ground, whereas true cables need reverse stockinette to set them off and would only produce a subtle, puckered effect on stockinette. As an example of the dramatic texture which RT and LT can form, take a look at this example of the Sunflower Tam from Norah Gaughan's Knitting Nature -- the mock cables are essential to this particular look. As you can see from this, mock cables are not a lesser technique than the true cables, merely a different one.