A faulty movement pattern is:
- when you perform a movement your body was not designed to perform (leg extensions)
- when you perform a movement incorrectly and therefore increase your risk of injury (knees cave inward during a squat)
- when you perform a movement incorrectly and therfore limit your potential (early arm bend during a clean)
Basically, it's when you teach your body to do something the wrong way.
One very important thing to remember is that when you repeat faulty movement patterns on a regular basis, your body will use those patterns when reacting to situations outside of the gym.
If you practise the deadlift incorrectly, what do you think happens when you try to pick up a box at home? You'll magically perform a perfect deadlift? Not likely.
If you don't teach the muscles in your legs and posterior chain to work together, what do you think happens when a situation in life demands it (tackle, run, jump, whatever)? That you'll magically pull things together and save the day? Not likely.
Whether you train "only for looks" or not, you still have to function as a human being outside of the gym. Teach your body how to move properly and you'll stay injury free for a longer period of time.