I find myself in agreement with the definitions listed on Climbing.com, so I have excerpted them here:
Backstep n, v : To press your shoe’s external edge onto a foothold and drop the knee lightly, thus bringing the sole’s bottom-outside in contact with the rock and your hip in; often opposed against your other foot’s big toe, off which you resolutely push. Unlike a Lolotte (drop knee), the less aggressive backstep typically exploits footholds below knee level.
Lieback n, v : To lean horizontally (sideways) off a hold, often a crack, and walk the feet high in opposition.
To me, simply sideways to the wall is not sufficient to call a move back-stepping. For example, if my right hip is against the wall, and I extend my right foot forward to place it on a hold, I would simply refer to it as using the "outside edge". But if I moved my right foot backwards to reach a hold, I would consider it a backstep.
I find it hard to confuse with a lieback, as the nature of the lieback is to have your feet in front of you, usually pressing against the same feature against which your arms are pulling.
On a related note, a question about what to call "leg behind rope" was good for about 5 pages on Mountain Project, collecting a variety of suggestions, some more amusing than others.