The viewport (also known as the 3D view) is where the user interacts with most 3D elements of the scene.
The viewport is where you can see and edit objects in Blender (3D View Editor). It is where the majority of work is done (including modeling, posing, painting, sculpting, etc).
It is just one of the many editors available in Blender.
Also see the viewport section of the Blender Manual.