These areas where the bones of the skull fuse together are called sutures. In total, there are around 20 sutures on the human skull. Although some of them have special names, most of them are named based on the bones they articulate. A couple of examples are the Coronal suture and the Lambdoid suture.
These sutures are made up of what we call Sharpey's fibers - a thin layer composed of collagen fibers that connects two adjoining bones.
Although these fibers connect two bones, they do allow small movements of the skull bones, causing the skull to be somewhat flexible (which is good for avoiding severe damage to the brain upon impact). Also, during the adult life of a human, the skull continues to change, although not as rapidly as that of a new-born. This in turn allows us to deduce the age of a human based on its skull (which is good knowledge for archaeology). But as the individual gets even older, these sutures may turn into bone completely (they become ossified). There are cases where these sutures ossify in a newborn skull. This is a condition called Craniosynostosis and it can severely affect the brain's development.