A rift valley is formed by extension within the crust.The crust is stretched and pulled apart and this thins the crustcausing the central area to drop down forming a wide broad valley landforn called a graben. If after further extension the crust becomes thin enough then volcanic activity can occur such as the East African Rift Valley (Kilamanjaro).
Continental break up starts with rifting and may eventually form a new ocean as the continent is pulled apart. Rifts do not always herald the start of a new ocean. The North Sea is a failed rift and graben which did not quite make ocean.