Can someone please explain co-op multiplayer?

I started "souls" games on Bloodborne, but I soloed every inch of BB without any notes (no PS+) or anything. I'm really not interested in PvP at this point, though (as I can get frustrated enough with PvE)

1. How do I start a co-op session, as both summoner/summonee?
2. When should I co-op? Farming? Just bosses?
3. What do I get for being summoned vs. summoning?
4. Is this a good way to farm souls if I am having a hard time progressing, or should I stick to "farming runs"?