A group is an ordered set of graphic objects which behaves as one object from outside. It is selected or transformed as a whole, or you can open a View on it and manipulate its contents individually. The selection in a group is independent of other groups.
The group structure of a page is tree-like. A group can contain multiple groups and/or individual graphic objects in it. The order of the objects inside a group is known as the z-order, the newly created objects are positioned to the top (the start) of the group. A group is atomic in the z-order of its superior group. To learn how to edit the z-order, see Groups and paths.
The group hierarchy is independent of the geometric dependencies. An object can be dependent on another object in a totally different group. Anyway, we do not recommend you to create very complicated dependency relationships which cross the group hierarchy many times, as you might easily lose track about what you have created and which object is dependent on which one.
A page is a special case of a group. The page's default group is called the top-level group. When you create a blank page, it contains the top-level group only. Top-level groups are not visible in the Universe browser and you cannot delete, ungroup or manipulate them.