WPF UI is represented in XAML which is a XML format. In XML elements are arranged in a hierarchal fashion. For example if you look at the below XAML (it has been downsized for simplicity) we have a Window element, the window element has a Grid control and the grid control has a button inside it.
So if you visualize the above XAML logically you can think that the button control is a child element of the Grid and the grid is the child element of the Window. This relationship between the elements which looks logical looking at the XAML is termed as “Logical tree”.
But now to display this Logical tree on to your screen you need lot of visual elements. like border, text etc. So when you add these visual elements to the logical tree that complete structure is termed as “Visual Tree”.
Putting in simple words there is only tree in WPF but depending on how you view it these two trees are the outcome. If you use the WPF visualizer the above XAML tree looks something as shown in the below figure which is actually a complete visual tree.
In simple words whatever you see in your XAML is a logical tree and to make it display it uses the visual tree. Below is an in detail figure which shows the logical and visual tree of the above XAML. Logical tree is without the shades and with shades is the visual tree.
Below video explain what is WPF and how they differ from Windows form