Squat is the most important and most effective exercise for strenght building, therefore this is the most important exercise for vertical jump development. Due to hip and/or ankle immobility not everybody can make full squats with correct form. They have to choose one of the alternative exercises like half squat, front squat, etc..
Useful links for squat:
Barbell Squat
Full Barbell Squat
Squat Analyzis
Here is some thorough article about squatting:
How to Squat (by DieselCrew)
Barbell Back Squat (by VerticalJumping)
Squat and Deadlift (by VerticalJumping)
How to Master the Squat (by StrongLifts)
