In the admin interface for a given model the __str__() or __unicode__() methods can be used as string representation, as they are valid in list_display as any other model method.
list_display = ('__unicode__', 'other_field')
Those fields are not database fields and cannot be used in sorting (because sorting is done at the database level).
However if an element in list_display represents a certain database field, it can be used by indicating it via setting the admin_order_field attribute for this element.
In such sorting can be enabled by defining an extra method:
# admin.py ... class Admin list_display('uni_sort', 'parent', 'name', 'slug',) ...