Camera aim is dependent on the targeting state the camera is in:
Free
A free camera's aim direction is dependent on its target position.
You can modify this position when the move tool [W] is activated or via the inspector.
Targeted
A targeted camera aims directly at a marker in the scene.
Targeting can be a quick way to modify the directions of many cameras at once.
A free camera can target [a marker] by selecting the camera(s) and pressing Set Target Marker, then selecting the marker to use as the look constraint.
A targeted camera can be unconstrained by selecting the camera(s) and pressing Remove Target.
Roll can be modified via the Roll field when a camera is selected.
You can also View Through a selected camera, and modify the roll by dragging the circle in the camera's view.