There are a combination of coverings on the skull. Firstoff, the face and skull base (the underside of the skull) are covered in muscles, vessels, nerves and skin.
The forehead area has a muscle called the Frontalis, which is then covered by some layers of connective tissue and skin. The sides, or temples, have one of the muscles of chewing (Temporalis) on each side, which radiates upward and backward on the skull. The back part of the head has a thin muscle called the occipitalis, below which are some big bulky muscles which hold the head in place on the neck.
The top part of the head in between (maybe the size of your hand, give or take) does not have a muscle covering, but it has a thick band of connective tissue called the Galea Aponeurotica, which connects the Frontalis, Temporalis and Occipitalis muscles together.