re: what makes an assault rifle an assault rifle?

Thanks Bayoudude for a great answer. Every part on a modern hunting rifle has evolved for a specific reason. Longer barrels equal better accuracy etc. etc...What are the reasons for the design charecteristics of what most refer to as the modern "assault rifle"? Why is the stock shortend? Why is the magazine high volume? Why is the barrel shorter? Why are there fore end grips on many? I'm sure there are other charecteristics as well. What functions or needs in performance of the "asault rifle" type weapons has influenced specific design charecteristics of them to evolve into something that so distinctly distigushes them from a modern deer rifles?