A folder acts as a way to group or contain lists, much like a folder on your computer contains documents. You can have as many lists as you like in one folder, but a list cannot belong to more than one folder.
In the example, the folder is called "Office". The lists "Management", "Our Project" and "Unser Projekt" are assigned to it. The mentioned lists are assigned to this one folder only.
A list is a simple list of tasks. You can have as many tasks as you like in one list.
You can add members to folders or to lists to share their contents with them.