Where two tectonic plates meet, this space is called a boundary. Depending on the type of crust involved and the direction of plate movement, different scenarios can occur.
For example, when oceanic crust and continental crust move toward one another, the greater density of the oceanic crust causes it to sink beneath the less dense continental crust. Eventually, the oceanic crust melts due to the heat within the asthenosphere and becomes magma. The magma rises under the continental plate and forms volcanoes. The sinking crust makes oceanic trenches which are very deep areas within the ocean. I have described a subduction boundary in the previous example.
A collision boundary is where two continental plates move toward one another and the force and pressure build up causing the plates to buckle and form a mountain range.
In a diverging boundary, two plates move apart. This occurs on the sea floor and is known as sea floor spreading. Magma will rise up and fill in the gap as the two plates separate. This forms a mid-ocean ridge.
Another boundary is called a transform boundary where two plates slide past one another. This is demonstrated by the San Andreas fault, in California. Here, the Pacific Plate and North American Plates are sliding past one another and the fault is a crack that has formed between the two tectonic plates.
I have included a link to animations showing plate tectonics. These are valuable as visual aids to further illustrate the answer provided above.