There are many variations of squats and many forms of resistance that can be used. Obviously, any squat incorporates your own bodyweight, and this alone can be a good stimulus for the beginner learning the movement or for anyone getting warmed-up.
When bodyweight squats stop providing the demand we need to expand, it's time to add resistance.
Kettlebell, Dumbbell, Sandbag Front Squat
I truly love these because they can be performed anywhere, are easy to learn, and even easier on the lumbar spine. Hold the weight close to the chest right below the chin. Not only does this position hit the quads but also forces the trunk to stabilize with brute force.
Barbell Back Squat
Place a barbell on the upper back and resting on the trapezius muscles. If the barbell is uncomfortable, you gotta build those traps, known as the "top shelf". The barbell should never rest on the cervical spine. Barbell pads exist, but I say work on those traps instead.
Barbell Front Squat
A great alternative to the back squat because it places less stress on the lumbar spine. However, the placement of the barbell is more complex because it rests on the shoulders with the hands holding it in place and the elbows as forward as possible. It can place quite a stretch on the wrists, but many lifters use the front squat in their training regimen.
Barbell Overhead Squat
A very powerful and admirable squat, the barbell is held overhead with arms fully extended. The core strength required for this movement is huge and well worth the practice. Even a broom stick held overhead is a beneficial start. Trust me.
Squats and the Knees
Bottom line: If you have a PRE-EXISTING knee condition, squats may aggravate this condition. Squats may also strengthen and improve this condition. This isn't known unless you give it a try with bodyweight squats and progress from there. Those with healthy knees can enjoy a lifetime of squats as long as resistance and form are always monitored. I'll end with this quote from top trainer, Alwyn Cosgrove. "Squats are not bad for your knees; it's the way you squat that is bad for your knees."
Make it a great day!