To add to what Fango has said- a professional image may also have been subject to some serious post-production editting, photoshopping etc.
To show just what a difference this can make to an image, check out this- its only about a minute long.
http://www.youtube.com/watch?v=pcFlxSlOKNI
A sign of this is that the model looks just a little unnatural- skin unnaturally smooth and belmish free etc.
Here is a "before and after" example- same person, taken at the same time- but notable differences.