Working into the back leg or loop twists the stitch. This is a quick and easy way to change the way the stitch lays in your knitting and is often used to create right or left leaning stitches. Back leg and loop are used interchangeably in this context–and I find myself doing it just from laziness–but it is technically more descriptive to say leg.
Back Leg or Back Loop?
When we are creating a stitch it is essentially a loop over a needle. It seems to me that loop describes the entire structure or the portion of the structure we see. The legs describe the two sides of the stitch or loop. Of course either is okay, and it is only because I want to be clear that I see any need to be more precise.
In the sketch above you can see the parts of the stitch or loop. There is a right leg, and a left leg. On the needle, see below, the right leg is the front leg, and the left leg is the back leg.
To support my usage, let’s look at dictionary.com’s definition of leg:
noun
1. either of the two lower limbs of a biped, as a human being, or any of the paired limbs of an animal, arthropod, etc. that support and move the body.
. . .
3. something resembling or suggesting a leg in use, position, or appearance.
. . .
6. one of the sides of forked object, as of a compass or pair of dividers.
Loop describes, but when giving directions it is clearer to be able to specify a specific part. dictionary.com’s definition of loop would lead me to believe it is a structure, not a part:
noun
1. a portion of a cord, ribbon, etc., folded or doubled upon itself so as to leave an opening between the parts.
2. anything shaped more or less like a loop, as a line drawn on a paper, a part of a letter, a part of a path, or a line of motion.
3. a curved piece or a ring of metal, wood, or the like, used for the insertion of something like a handle, etc.
In Eastern Crossed or Eastern Uncrossed, the purl creates a twisted stitch. Eastern Crossed leaves that twist on the knit side by working a regular knit (into the crossed stitch), while Uncrossed knits into the back leg to untwist the stitch on the right side. The textile you are creating determines whether you need to uncross. The cross is usually created by how the purl is wrapped.
Most knitting instructions assume Uncrossed knitting and show the open bottom of the stitch of a regular knit stitch.
Working into the Back Leg
Stitch patterns calling for a crossed stitch ask us to work into the back leg or loop. This causes the stitch to lean to the left and crosses the bottom of the stitch. Purling into the back of a purl stitch twists it in the same way, but it leans to the right on the right side. It is very hard to see any twist on the purl except on the reverse side.
If you find it awkward to knit into the back, you can manually move the stitch so the front leg is twisted to the back and then work into the twisted stitch. Same for working into the back of a purl.
Calling Out the Front and Back Leg
Once the loop or stitch is off the needle, the front leg becomes the right leg and the back leg becomes the left leg. If the stitch is twisted at the bottom, what would be the right leg becomes the left leg.
I like to think of a stitch or loop on the needle as a frog cowperson sitting on a fence: The frog sits comfortably atop the fence and is ready to jump off, jump up, or perform whatever action is needed. If a cowperson frog is sitting with its legs crossed as a twisted stitches legs are, and facing the same direction, it will be exceedingly difficult to sit comfortably atop a fence. 
Working Two Together in Back Leg
This is done nearly identically to working the single stitch. K2tog-b just goes into the back leg of two stitches, and is worked. Remember, a k2tog leans to the right, so working through the back leg causes a left lean. P2to-b is worked going into the second stitch, then the first stitch, going into the back leg from left to right, behind the needle. This can be quite awkward. Manually moving the stitches on the left needle and then p2tog reduces the difficulty. If a p2tog leans to the right, then the p2tog-b leans to the left.
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