Blender provides a number of different editors for displaying and modifying different aspects of data. An Editor is contained inside an Area which determines its size and placement within the Blender window. Every area may contain any type of editor.
The Editor Type selector, the first button at the left side of a header, allows you to change the Editor in that area. It is also possible to open the same Editor type in different areas at the same time.
See User Interface for documentation on the general interface.