The 3 types of tectonic plate boundaries are convergent, transform, and divergent.  A convergent boundary is one where two plates are moving toward each other.  Typically one plate is forced beneath the other (ex: the Andes mountains and the Japanese islands).  A transform boundary is one were two plates move past each other.  One in a sinistral motion, and the other in a dextral motion (ex: the San Andreas Fault).  A divergent boundary is characterized by two plates moving away from each other.  A fissure is formed.  Magma enters the fissure, forming new crust.  When divergent boundaries occur in the ocean they are termed "oceanic divergent boundaries."  When they occur beneath continents they are termed "continental divergent boundaries."  Examples of divergent boundaries are the Mid-Atlantic Ridge and the Great Rift Valley. 

Iceland is an example of a country undergoing a continental divergent boundary.  The Mid-Atlantic Ridge divides Iceland and is the boundary between the North American and Eurasian tectonic plates.  The North American plate is slowly moving westward; while the Eurasian plate is slowly moving eastward.  The country of Iceland is slowly being pulled apart.

When divergent boundaries occur under continental plates, they form a rift valley. Initially, the valley is fairly narrow, but it still allows rivers or streams to flow through.  At one point, it will dip down below sea level, allowing a sea to form.  Given the right conditions, and enough time, new ocean basins may even form.

The East Africa Rift Valley is an example of a continental divergent boundary.