Layers can be grouped together for easier management and for restricting adjustments or masks to particular layers within your project. Layer groups can be nested together within a 'parent' layer group.
Grouped layers remain together so they can be easily selected, moved and copied. Furthermore, once a layer group is established, adjustments and masks can be applied to the group and will affect all the layers within the group but not those outside the group.
Groups, and nested groups, can be broken apart at any time into their separate layers.