ControlTemplate vs DataTemplate

When to use ControlTemplate and when to use DataTemplate/HierarchicalDataTemplate
Quick Answer:
- ControlTemplate is used to replace the whole visual tree of a control.
- DataTemplate/HierarchicalDataTemplate: is used only to replace the content of a control.
Example at the bottom will clarify more.

Features of each one of them:
ControlTemplate:
- you can use template-binding to bind some of the elements' properties defined in your template with the templated control's properties. Like to bind a rectangle's color in your template with the templated control's background color.
- you can use ContentPresenter to display the templated control's Content somewhere in your ControlTemplate (in case your templated control is ContentControl control).
- and you can use ItemsPresenter to display the templated control's items in case your templated control is ItemsControl control.

DataTemplate/HierarchicalDataTemplate:
- you can bind the elements you defined in your DataTemplate with properties of your corresponding Data (not control's property) classes.



Example:
if you have TreeView control and you have TreeViewItem(s) in it. The TreeViewItem is going to have some GUI in it to display the following:
- plus/minus button.
- border.
- place to display the header of the item, may be text block.
- place to display the child items of it, may be a stack.

if you use DataTemplate/HierarchicalDataTemplate, you only need to worry about displaying the text block (Content); so you don't have to worry about the plus/minus button nor to worry about the child items.

On the other hand, if you use ControlTemplate for this TreeViewItem, you will have to draw the plus/minus button and where to display the textblock (ContentPresenter) and where to display the child items (ItemsPresenter). and then you can add your own decorations around them.

I will keep updating this post as necessary. Please, let me know what you think and if you have any questions or concerns.