In regular perspective projections, for a point at (x, y, z), the point it's drawn on your screen is (x/z, y/z), translated to the screen center, and then maybe multiplied by a constant factor to zoom in or out. In an orthographic projection, the point (x, y, z) is simply drawn at (x, y) (and also translated and/or scaled by a fixed amount). If you draw a line that goes straight into or out of the screen, you'll see it as a diagonal line pointing towards the center of the screen in a perspective projection. In an orthographic projection, it will appear as a point.
(Edit)Here's a good image of an orthographic projection of an object: (url="http://"http://www.m-w.com/mw/art/orthogra.htm")http://www.m-w.com/mw/art/orthogra.htm(/url) (/Edit)
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(This message has been edited by Aprosenf (edited 01-03-2004).)