The group tool lets you group /ungroup objects.
Group: Convert the selected objects into single object ,This new created object has new attributes under current setting (trasnform,Transparency and FX),

Ungroup: Convert the grouped object into original single object(s),all objects re-use original attributes.

To use Group tool, select the objects you want to group, select group  tool from the system Tool Panel .

To use Ungroup tool, select the grouped object you want to ungroup, select Ungroup  tool from the system Tool Panel .