The integumentary system is a fancy term that basically refers to all the parts on the "outside" of the body. These parts -- hair, nails, and skin -- protect the body from outside damage. In addition to protection, the integumentary layer contributes to waterproofing and temperature control. The largest part of this layer is the skin. In fact, the skin is the largest organ in the entire body. The skin itself is comprised of two layers of tissue: the epidermis and the dermis. You can think of the epidermis as our shell. The dermis is a thicker layer that contains glands, follicles, nerve endings, and more.
The purpose of skin and all its appendages might seem simple, but it is actually more complicated than it seems. It not only serves to keep infectious organisms out but also to keep hydration and fat in. Another important purpose of skin is to synthesize sunlight into Vitamin D.
Like other systems of the body, some conditions can specifically target the integumentary system. Things like rashes, sunburn, acne, and eczema are examples of afflictions that occur specifically in the integumentary layer.