If you guessed the one on the left is the squat that sucks, then you're wrong and you suck. haha. No I'm kidding, you don't suck. But prepare to get schooled, son. The squat on the left is the one that you always hear about, how it's so good for your knees, hips, pelvic floor spine, etc. If you're doing the squat on the right, you're never going to see any of these benefits.
First lets compare the form. In a good squat, the shin bone will be vertical, the feet will be wide and flat and the outside edges will be parallel. The tailbone will be untucked, the torso fits between the knees, and the entire spine extends, right up to the crown of your head. This lengthens the hamstrings and calves, and requires you to use your glutes to hold yourself up (it's a great butt "work out"). It helps tone the pelvic floor, and maintains hip mobility. It doesn't overload the knees, and it doesn't open the spine in a dangerous way.
The bad squat allows the heels to lift, the knees to track over the toes (non-vertical shin) the tailbone tucks under (like the dog you yelled at on your lawn that time) and the spine curves forward, causing the neck to compensate by curving in the opposite direction. This overloads the knees, doesn't do a damn thing for your pelvic floor, doesn't really work on hip mobility, it opens the spine dangerously (especially for all those people who sit a lot) and it causes unhealthy disc compression in the neck. boo!!
So there you have it. That's why one is good and one is bad. Someone once asked me "but, isn't it better to squat rather than not squat, no matter what your form looks like?" My answer would be no. A squat is not really an exercise that you should do, but rather a movement you should be capable of doing, and one that you use multiple times throughout the day when you need to get down to the ground for something (maybe to pick up the present the dog left on your lawn). When you can do a proper squat, you are in REALLY good shape, alignment-wise :) Instead of saying "well, this is the squat that I can get into, so I'll just keep doing it this way" you need to pull back and do some stretches that will help you be able to do a full squat. Start here, with this awesome tropical squatting video by our favorite Katy Bowman.
Then head on over to her blog to check out some other sweet squat stuff that you can do to get your body back to where it needs to be :)