Hey there… Welcome to the forum…
Short answer is that Ward is just an additional shield above Health - i.e. it functions like more health and damage is dealt to ward first, then to Health… Unlike a normal “shield” ward decays over time, so unless you continually regenerate it (manually or through automatic mechanics from skills/gear), it will eventually dissappear.
Acolyte builds are currently able to generate a lot of ward… (so much its likely to get a nerf at some stage) but its generally a good mechanic for defence for this class.
Note - At higher levels, more Health usually trumps other kinds of defences due mainly to diminishing returns. If you want to see where you build would most likely gain the most benefit, then you can use Tunks EHP Calculator.
I recommend you check the Community Game Guide as well as press “G” while in-game for the built in Game Guide.
Specifically the Health & Ward sections for detailed information on the question…
I expect that if you are asking this kind of question, the Community Game Guide will provide a lot of answers to other things you have been wondering about…