Bodyweight, barbell on back, barbell in front, and so many more – squats are a versatile exercise for strength and flexibility. There are lots of variations depending on how your body is set up – some people like a wider a stance, some narrow, for example, and people with very long bones may find their knees do go past the toes – but this is a pretty good general guide. In the gym, the weighted squat is pretty technique-oriented, but the movement has a very natural side as well.
This particular image got criticized recently by Alex Viada – a competitive powerlifter and triathlete (a true unicorn). He doesn’t like it because a child’s proportions are so different from an adult’s, so it’s not a good guide for cueing or coaching an adult squatter. (Also, a child’s flexibility is vastly greater.) It’s still a fun illustration, and perhaps a reminder to “use it or lose it”!