There have been many questions about squat form and issues.

Lower back rounding and the bottom of the squat
GMing out of the Hole
Engaging the glutes and hamstrings, Part I
Engaging the glutes and Hamstrings, Part II
Bar positioning and Squat Depth
Hip and hamstring mobility and flexibility
A dozen selected exercises for squatters, part I
A dozen selected exercises for squatters, Part II
bands and chains
shoes
JV squats and overhead squats
setup and breathing
belts, wraps, and suits, Part I
belts, wraps, and suits, Part II
common issues, part I
Common Squatting issues, Part II

Thanks to GqArtguy for compiling this.