Roofs come in all kinds of shapes.  This gives good examples and illustrates the parts of a roof. 

A properly ventilated roof will last longer. It will help prevent the build up of moisture which can lead to molds and mildews.   

Not all shingles are created the same.  some are better and some are not.  Your budget, and your future plans for the house help with your choice. 

I can not vouch for their price estimates, but the flat roof material discussion is pretty good. 

Not an endorsement simply a good example of what a major supplier feels is needed. 

Still not an endorsement but this is what Mr. GAF thinks you should look for when evaluating your current roof for a replacement.