The brain is surrounded by a a fibrous membrane known as the dura mater (stands for tough mother in Latin).  The dura forms several "folds" within the skull, and these help to partition parts of the brain from other parts, and to help provide some stability to the soft and delicate brain. 
The falx cerebri separates the two cerebral hemispheres.  The tentorium separates the top portion (cerebral hemispheres) from the bottom portion (cerebellum).  A number of venous sinuses run within the folds of the dura.  These include the superior sagittal sinus, the inferior sagittal sinus, the cavernous sinus, straight sinus, transverse sinus, sigmoid sinus, and circular sinus.