Trees questions allows selection on a treeview. Trees can be eitheir OPT Trees (one item selected) or CHK Trees (multiples selections).
Trees can be Flat (up to 5 levels). Thats mean that hierachy is fully defined in the row (for example: Continent, region and country).
Trees can also be hierachical. That's mean that each row contains the ID of the Parent Row (for example, an employe row contains the ID of the manager of that employee)