2. Basics of Image Processing / 2.3 Layers
Groups are a union of several layers. The advantage of a group is, that all layers in-group can be moved or scaled at once. Also, groups are helping to keep the structure of a document simple, because fewer layers have to be considered.
The layers inside a group are also named "children". The children of a group are not directly accessible. In order to edit them, the group has to be entered (see chapter 4.7, chapter 6.2.1 and chapter 7.5.43). After that, the layers inside this group are accessible. If you have finished your work inside the group, you have to leave the group afterward (see chapter 7.5.44).
Groups can contain other groups, too.