CL_SALV_TREE 

The CL_SALV_TREE functionality is similar or basically the same as what can be achieved by cl_gui_alv_tree but does not require you to create a program screen to implement the functionality. This can make it a little easier to learn and build your ALV tree report and is more OO based, think of it as a kind of object orientated wrapper for the cl_gui_alv_tree screen functionality.

CL_SALV_TREE demo programs 

Within your SAP system, there is a suite of programs starting with SALV_DEMO_TREE* that demonstrates much of the functionality available and how to implement it

SALV_DEMO_TREE_DATA_UPDATESALV_DEMO_TREE_EVENTSSALV_DEMO_TREE_FORM_EVENTSSALV_DEMO_TREE_FUNCTIONSSALV_DEMO_TREE_METADATASALV_DEMO_TREE_SELECTIONSSALV_DEMO_TREE_SETTINGSSALV_DEMO_TREE_SIMPLE