View Cube
The view cube is displayed at the bottom right of the viewport and shows a graphical representation of the orientation of the camera in 3D space. You can interact with the view cube to align the camera to look against the direction of the selected axis. You may also toggle the lens projection of the camera.
Camera Direction
Left clicking on an axis of the view cube aligns the camera to look against the direction of the selected axis. For example, if you select the +Y axis, then you will be looking in the -Y direction.
Camera Projection
Right clicking on the view cube allows you to toggle the lens projection of the camera between pespective and orthographic. The orthographic view can be useful for precise positioning of parts.