Sections and groups are very similar in some respects. They both allow instructors to manage subsets of students within a site. For example, both groups and sections allow instructors to filter and view one section of students at a time in the Gradebook. However, sections and groups also differ in several key ways.
Sections are subsets of site participants. A section may have a variety of data attached to it, such as category, days of the week, times, and an assigned teaching assistant (in a course site only). Depending on how the software is implemented at a given location, the Section Info tool may be loaded with official course sections. Alternately, an instructor may be able to set up sections manually.
Groups are also subsets of participants for a given site. However, groups are not preloaded with official course data, and are created by the instructor or site owner instead. Groups are useful to organize study groups, project teams, and other, non-official subsets of site participants.