The difference between the overhead squat and the back squat lies in the placement of the barbell. During the overhead squat, the barbell is held with straight arms extended toward the ceiling. The back squat calls for the resistance to be resting on the top of your back. The placement of the bar changes several components of the exercise.
Both types of squats work your core, glutes and legs, but the overhead squat also strengthens your arms, shoulders and upper back. Holding the barbell over your head does make the overhead squat the more challenging of the two since more muscles, including your core, need to be engaged to keep your body stable. You should fully master a traditional or basic squat before performing the overhead squat.
The back squat movements are similar to a basic squat with the main difference being placing the barbell on your upper back. A greater amount of resistance can usually be used in the back squat compared to the overhead squat since your body is in a more simplified position. More resistance can lead to greater muscle gains, though at the same time, increasing the weight means following proper form to keep your lower back injury free.