Question on "Access levels" and "user and groups security"?

Hi All,

I am working on BO security and access levels and come up with very confusing situation where I need to know

1. What is the difference between User Security and All Access Level Security in Access Level Secuirty?

2. What is the difference between User Security, All Users Security and All Groups Security in Users & Group Secuirty?

I searched in BO admin guide but no luck.

Product using BOE 3.1

Any help appreciated.

Thanks,

Praveen.