This is a functional change to the way Wordpress operates. There is no material difference to the front-end of the website. It is an administrator function only.

A new way of segmenting the members has been added. This replaces the previous inefficient mechanism of using User Roles, which was very limited. User Roles were never intended to be used in this convoluted way.

The Groups plugin introduces a way of creating an unlimited number of user groups. These user groups can be used to select a set of users for a particular task, or access. Each group can have its own set of capabilities. Groups can be hierarchical, inheriting the capabilities from the parent.

The groups are easily assigned manually to users in the All Users page. Each user can belong to any number of groups. Most selections are now based on a single group (and should be), rather than multiple roles.

The front-end user profile shows the groups a member belongs to.

All previous user selections using roles has now been replaced.
All the page selections based on User Role are now set as access for everyone. Instead, each page has a new Groups selector.
All menu items are now set to access for everyone. The menu visibility is controlled by the page access rules. Any previous menu items which used custom links have been replaced with pages.

The one downside is that selection by role was often included in other plugins. Instead, the ASC plugin has been extended with custom functions to work with groups instead of roles.