Can Someone Please Explain?

Sorry for my non-understanding-ness, but can someone explain to me how the element system and identification system now work? Like when I hit a mob and it says, -(number) and it's yellow for thunder damage? I'm just not exactly understanding. And like how I identified my endgame (or so it was) armor and gave me all those stats. Explanation??

Also, I know this is a very stupid question due to the new systems, but could someone advise me on what and how to build classes? Like what stuff to go with for an Assassian or Archer (stupid, I know)