Unfortunately, knee injuries are common amongst both athletes and the general population. That has created a huge market for suppliers of support products, who offer a variety of different knee supports.
The most basic knee supports are pullover neoprene or elasticated supports, with either an open or closed patella. These offer a basic level of support from compression of the joint.
An alternative to this style are the knee wraps. These have the advantage that they can easily be fitted to swollen or injured knees without any pulling or tugging. With Velcro wrap bands, you can also customize the level of support to different parts of the joint.
Some supports have more solid components to increase the support they offer. You'll find neoprene sleeves with flexible spiral stays for added medial/lateral stability, and also a removable horseshoe that can be placed in various locations depending on where support is needed.
Hinged supports offer the greatest level of protection. These can help reduce medial or lateral instability, and also protect against side impact. In addition, the hinge locks to prevent hyperextension of the joint. Many people wear these during sports such as football, hockey or skiing.
Finally there are patella straps, worn to treat pain from conditions such as Chrondromalacia, Patella Tendonitis, and Osgood-Schlatter's disease.