What is the difference between a user and a role?

I know there are other threads that are similar, but I am not sure if they are relevant to Postgres.

I am reading the PostgreSQL documentation which it reads as follows:

Note: As explained in Chapter 20, PostgreSQL actually does privilege management in terms of "roles". In this chapter, we consistently use database user to mean "role with the LOGIN privilege".

Does this basically mean a role is a database user? Or is there a difference between a role and a user? Do users have the potential to not have full privileges while roles are users who always do have full privileges?

Previous versions of Postgres, and some other DB systems, have separate concepts of "groups" (which are granted access to database objects) and "users" (who can login, and are members of one or more groups).

In modern versions of Postgres, the two concepts have been merged: a "role" can have the ability to login, the ability to "inherit" from other roles (like a user being a member of a group, or a group being a member of another group), and access to database objects.

For convenience, many tools and manuals refer to any user with login permission as a "user" or "login role", and any without as a "group" or "group role", since it is useful and common practice to keep roughly to that structure. This is entirely a convention of terminology, and to understand the permissions, you need only understand the options available when creating roles and granting them access.

Again purely for convenience, Postgres still accepts commands using the old terminology, such as CREATE USER and CREATE GROUP which are both aliases for CREATE ROLE. If you write CREATE USER, the LOGIN permission will be added to the new role by default, to emulate the old behaviour when that was a separate command.

I found this link pretty useful. The final goal is that some user/role can readwrite, some rule/user can be only read.