What is a grouping?
A grouping is a collection of groups. When you have made groups in a course you have the option of adding them to a grouping. A single grouping can house one group or several groups.
Note that since you can have a group of only one member, you can therefore have a grouping of only one member too, if you wish to restrict an activity to a single person in your course.
Groupings were designed to allow a selection of different sets of groups in each activity. See Groupings for an example scenario.
What is the difference between a group and a grouping?
A group is a set of users. A grouping is a set of groups.