Sections are used to group rules, actions for each extension using the table jos_access_sections
These are users stored in jos_users table. Please note that gid and usertype fields are only there for legacy purposes and are not used in the current ACL system. Users can be mapped to rules via jos_user_rule_map table. in phpGACl, users were called AROs (Access Request Object)
These are user groups that are hold in table jos_usergroups. You can have nested user groups. Each group obviously can hold an unlimited number of users and each user can be assigned to an unlimited number of user groups. These relations are hold in the table jos_user_usergroup_map. User groups can be mapped to rules via jos_usergroup_rule_map table.
Actions are things your users will perform such that logging in to backend
Assets are items that you need to set access control on. For example each article on your site can be an asset and you can set edit permission for them. Currently these are not used in core.
These are used for creating different view permissions for a combination of usergroups. (???) How this is achieved:
Rules are combinations of actions and usergroups (or users) and optionally assets There are three types of rules: