Here is a really simple explanation between the two. Hope it helps. A perspective view is going to be more realistic to what our eyes see. Objects are relative to each other and will not always create 90 degree angles. As you get further away from two objects, those objects will seem closer together. In Orthographic, all things are constrained to set distances. So the further you get from two objects, they will always look the same distance apart. This is a more flattened field of view and is good for more of the cad style look.

This page offers additional aspects and images! Orthographic projection is a form of parallel projection.

http://en.wikipedia.org/w.../Parallel_projection

Thanks for replies. The reason I ask, is that my realtime visualisation seems to be lagging somewhat when Im in perspective mode