This article will give you the basics of and differences between the three entities Groups, Groupings and Cohorts
Using these entities can sometimes get a bit confusing and this article will help you to understand the context and basics.
Let us now start with Groups!
GROUPS
To begin with, Groups can be created only at course-level.
It allows you to group the users within a course.
The users enrolled within a course can be split into separate groups. It is especially great when you would like to separate the users into separate groups and enforce them to interact only within their group peers.
Another great place to use this would be when you like to have certain resources or activities available only for a certain set of users. You can put these users within a group and open the activity or resource only for that group.
Group discussion forums will allow you to create discussions within each group.
There are three group modes for course activities/resources.
1. No groups: There are no sub groups, everyone is part of one big community 
2. Separate groups: Each individual in a group can only see their own group, other groups are invisible to them 
3. Visible groups: Each individual works in their own group, but can also see other activities and resources from other groups (work is read-only and they cannot participate) 
GROUPINGS
Groupings are also created only at the course level. Grouping allows you to facilitate one or more groups to work together.
Moodle definition for Grouping is as follows: A grouping is a collection of groups within a course. Using groupings allows you to direct tasks at one or more groups in your course so that they can work together on the tasks.
To have students working on different activities, not even seeing activities for other groups (such as group A writing a report, group B using a forum, and group C creating a wiki), you need to put them in groups first, such as group A, B, and C, THEN you must place each group in their own grouping, such as Grouping A containing Group A, Grouping B containing group B.
The only way you can make an entire activity available for only one set of students is to pass through groups to groupings. This allows you to customize the same course by presenting different content to a different set of users.
Need step by step instructions on Groups and Groupings? Use the links below.
How to create a grouping and add groups to a grouping
How to assign users to a group
COHORTS
Cohorts were previously called as system-level groups.
As the name suggests Cohorts are created at system-level, unlike groups and groupings.
The intention of cohorts is to have a whole class of people into a group so that they can be easily enrolled and un-enrolled from courses.
Cohort sync is for automatically enrolling members of a cohort in a course.
If a user is added or removed from the cohort, they are automatically enrolled or unenrolled respectively. In other words, cohort sync synchronises cohort membership with course enrolment.
Cohorts are visible in role assignment pages just like people, you can enrol a cohort in a course all with a particular role.
Now sure about how to create cohorts? Use the links below.
Create a new Cohort and Assign Users to a Cohort
Enrolling users into a course via Cohort Sync
If you've used cohort sync (which automatically synchronises cohort membership with course enrolments), deleting a cohort will result in all cohort members becoming unenrolled from the course(s) that the cohort was synced with.
Some data associated with the members' activity in the course(s) will also be deleted.
If you've only manually enrolled a cohort in a course, then nothing happens if you then delete the cohort - users remain enrolled in the course.