A Widget is a display element the administrator can place into a Container in order to display something specific at this place.
There are many different widgets available and each can be configured in a variety of ways.
Skin designers only add widget containers into their skins. The actual widgets are then placed into the containers by each collection owner using that skin. (Note: when creating a new collection, b2evolution populates common containers such as the Header and the Sidebar with some common widgets).