There is no better way to understand any complex subject than learning via diagrams. Security is very complex and dry subject. Not everyone understands it easily. I have seen quite a few of the developers who are good at performance tuning or other database concepts, but when it is about security, they are not so comfortable with it. Microsoft has recently released SQL Server Database Engine Permissions Poster. These posters are a great way to understand what are different permissions required do various tasks.
For example, if you want to do Audit on the server, you may need control server and database audition permissions. Here is a screen shot of the very small section of the white paper.
I think there are plenty of goodies in the poster. Download the poster from here. In the same page there are plenty of good posters available. Everyone should have this in their workplace.
Reference: Pinal Dave (http://blog.sqlauthority.com)