What does "cover" and "sneak" do?

[LobsterMagnet]

In the Concealment Tactics in the Rifleman tree, you get cover + 20 and the "sneak" ability, I was just wondering what those do. I was told by someone sounding intelligent that cover makes you harder to hit, but I wanted a 2nd/3rd/4th opinion

 

I was also thinking cover hides you from notice, and if that was true I assume that only works on NPCs...same thing with "sneak"