Ok, I wrote this from memory, with no pov nor doc to check it. I write 
only about the perspective camera. Some things apply to other camera 
types, other do not. (Very helpful, isn't it ?)

Perspective camera uses perspective projection. To understand it, the 
easiest is to go back to a real-world explanation : to paint as the eye 
see, paint what you see through a pane  of glass on the pane itself. 
That is, from the eye, trace a ray to any point you wish to trace and 
then colour the glass at the intersection of the ray and the pane.

location is the eye, the point from which all rays pass. up, right and 
direction define the virtual glass pane through and onto which pov 
traces, and which is then mapped onto the +w,+h window.

up and right define a parallelogram, most usually a rectangle (other 
shapes distort the picture). It's centre is placed at direction. up is 
mapped to +h and right to +w.

Usually, up, right and direction are orthogonal. If up and right aren't, 
it skews the image. If direction isn't orthogonal with up and right, 
while up and right are orthogonal, the effect is as if you traced a 
rectangular part of a larger trace. (There is a standard include files 
that does just this, I suppose it must be in a Camera directory.)

Now, look_at rotates the camera (the up, right, direction trihedron) 
around the origin to align direction with look_at. The camera is first 
rotated around the sky axis then around an axis normal to both sky and 
the new direction.

angle is the angle from both end of right (when it is centred around 
direction) to location. Changing angle is akin to modifying direction's 
length.

location, right, up and direction define the camera. sky, look_at and 
angle are tools to manipulate these fundamental constituents.


Does this help?


Povingly,

Philippe

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