Tablet shells are known as back protectors. They are designed with portability and basic defense of the device in mind. Shells only protect the back and not the front, so they are generally lightweight and inexpensive.
Most often, these are simple inflexible backers for the device which provide safety in transporting the device while giving extra support to the device. Sometimes back protectors are accompanied by a screen protector to fit over the touchscreen. A few manufacturers provide a bit of functionality to the shell in adding a stand to the back for viewing.
Regardless of material or shape, a tablet shell fits the device very snugly and contains additional holders inside to prevent tablet slippage or movement while carrying. Tablet shells are usually made of plastic, TPU, carbon fiber, and even wood or bamboo. Leather shells always use a plastic frame inside.
While a good tablet shell may offer well-above-average protection in its area, any prospective owner of this accessory must remember that the screen will remain unprotected.