I've been doing some reading on the front squat vs high bar squat vs low bar squat, and I have a pretty good idea of how the leverage system plays a role in how much weight we can use. But what exactly is the sticking point in a squat and how does that relate to the levers in the back squat?

It depends on the person. Often (especially with new lifters) it's at the bottom of the movement.

With respect to levers, if you think of the body mechanically, you're looking at two sets of levers based on where you are in the squat. You have levers that follow the hip and levers that follow the knee; so the lever through your back comes to the hip and the femurs levers up with the hip at one end and the knee at the other. Finally, bnetween the knee and foot you have what some call the "shank lever". See here for a solid analysis that uses the name 'shank lever': http://startingstrength.com/articles/squat_analysis_soleyn.pdf

So, depending on where you're stuck, one or another lever can't "activate" to do work to push the weight up. Sometimes de-loading solves the problem, but ultimately form correction is what you would want.

It's relative, dui cutie.

Thank you fellas. I wasn't aware that the sticking point was relative, but instead assumed it was when the thigh was approximately parallel in the squat.