Inner CV Boot/Joint — The inner cv boot (where my finger is point to) is connected to the cv (constant velocity) shaft which on the other end at the bottom of the picture is the out cv boot and joint. The purpose of the soft rubber boot is to protect the joint from road debris and keep the special lubricating grease inside the boot to lubricate the joint. The boot has to be soft and flexible because as you turn the wheels the boot stretches.
When the boot breaks the grease drips out and water and dirt get inside the joint causing damage. A bad cv joint will usually make a clacking noise on hard turns. it can sometimes be more economical to replace the entire cv shaft (comes with inner and out cv joints and boots) with a rebuild unit than it is to replace 1 bad cv joint.