Global transform mode defines a viewing position and then allows you to transform (translate and rotate) the scene. Camera mode behaves a little differently. In this mode the scene remains as defined and does not move. Instead the scene is viewed through a camera which can be positioned at any location and oriented to look in a specific direction and have a specified tilt.

A viewport can either be in Global transform mode (default) or it can be defined to be viewed through the camera. Local transforms are valid in either mode. In order to view a viewport through a camera (a) the camera position and orientation must be defined, and (b) the viewport must be tied to a camera.