There are many different examples, but I will provide some in my situation of a secondary school.
In summary, everyone who has an account is a user. Eg. all teachers and students are users.
Groups are groups of users within a course. For example, we have a language course used by all Year 8s, but within that course they are grouped according to which teacher they have.
Groupings are groups of groups within a course. I haven't made much use of this to give a practical example, except in some cases features require groupings rather than groups, so adding each group to their grouping might allow these sorts of things. I would love clarification from someone about this!
Cohorts are groups of users within the site. For example, "Year 10" or "Teachers" might be cohorts.
Hope this helps!