Ahhh squats. The mother of all workout.
To many people, they not might see the point of squats. I mean, we use legs walking around everyday, right?
Consider this:
Back then, people walk or cycle to move around. They have strong legs.
Nowadays, especially city dwellers, we walk less and drive more. Our limbs are getting weak.
Hence squats slowly creep into our workout menus.

The benefit of squats
Strengthen knees and thigh, lower back, reduce chronic knee pain, better balance.

How deep you want it to be:
Quarter squats – Thigh parallel with the floor
Ass-to-grass squats a.k.a deep squats – Lowest point your butt can go. Making use as much range of movement as possible.

Types:
Air squats
Single legs a.k.a pistols
Jump squats
Weighted squats
Front squats
Back squats
Overhead squats
Thrusters
Snatch
Clean & jerk
Single dumbbell snatch

How to do them:
Always feel the weight on the heel to midfoot. if you are doing squats without weights, yes, try to keep your knees before your toes and keep your shin vertical.
Back straight throughout.
With weights – It is ok to feel some weight on midfoot. You need to utilize as much muscle as possible during the heavy lifting, you will learn forward at some point of your lift.
Always make use to thigh and buttocks to pull yourself up. Those are some of the largest muscles on your body.
That being said, you will need to consult your physician before proceeding to anything other than air squats if you have spine injury or problems, as wrong posture may put a lot of pressure on your spine.

Tips of keeping your back straight:
Belly breathing and holding certain amount of air as you stand up. Let some air out when needed be, especially landing of weight after a pull of power pull, such as complex moves like Clean & Jerk.

Now go get your buns of steels!

picture credit to http://strikebrewingco.com