The term "repeater group" refers to a feature of the drag-and-drop editor. Repeater groups allow the inclusion of multiple rows of data in your content.
A customer places an order online to purchase 4 items. The design used for the order confirmation email contains a repeater group for product description, product image, product quantity, and product price. The repeater group used will "repeat" itself 3 additional times to account for the 4 items purchased.
How is it different than a cell group?
A repeater group IS a cell group. It simply behaves differently in that it can repeat itself where multiple rows of data exist.
Repeater groups are functional only in data relationship of one to many (multiple records exist per email address) and available to professional license or above.