Capistrano Permission/Users/SSH/Git

Question

I think I need to understand this from the basic level. Form what I know so far it seems, there are a few user account involved in deploymnent

1 - Deployer user for capistrano

2 - www-data account (not sure exactly what this does)

3 - the git user

What do ..

1 - each of these users do?

2 - what permissions they need onthe production server

3 - what ssh keys do I need to put in place so that I can do capistrano deployments without having enter my passwords all the time

here is my set up so far.

http://pastie.org/1189919

I have looked around and there doesn't seem to be any 1 comprehensive documentation that explains this.