How does combat work?

Sorry if this has been posted elsewhere.   I was curious if someone could explain how ship combat works in the game.  I've been a GalCiv player since the first game so understand the basic dynamics.  

Essentially, I am wondering what role Defense plays for ships.  In GC3, I vaguely recall that the Defense acted as a sort of HP role such that in a longer combat, the Attack rolls would eventually wear down the Defense and start taking away from actual ship HP.  Frankly, I didn't like this system as Defense was only useful to blunt attacks (there is more nuance but that was the general idea).  I would have liked a high Defense to always blunt weak Attack numbers (e.g. Attack Laser level 1 could never touch a ship with Defense Shield level 4). 

Any clarification on how it works in GC4 would be greatly appreciated!