Are any any differences between shield and structure point or do you just add both to get your hp?

There is a difference. Shield points represent how much your ships/defenses can absorb without being damaged. Structure points are more like the HP you're referring to. Remember, though, that any damage the ship takes once it's shields are overpowered can possibly destroy it. The more damage it takes, the higher that chance.

I'm by no means an expert on the workings of battles, but I'm sure someone will come in and elaborate on this answer.

Here are a few basic differences.

Shields: they work as combined shield while in a fleet for all ships. that is if u have a ship type with shield 100 and u have 1000 ship then the fleet has a total shield of 100.000, that it protects each ship in a whole.

Armor: armor in a battle is 1/10th of the structure point of a ship which is individual. in a battle each ship takes armor damage individually. and karr is right a ship can only take a certain amount of beating before it is destroyed. but to make a ship destro an attacker must make a minimum 30% armor damage to get in the chance factor of crashing the ship. any more damage just increases the chance of the ship getting crashed. (the 30% reference is an educated assumption, consider it as my personal understanding .. ze dev team can confirm)

Structure points are just a combination of however much Metal and Crystal it takes to build the ship to my knowledge.