The #type property specifies that the array contains data that needs to be rendered using a particular render element. Render elements are plugins (yes, plugins) that encapsulate a defined piece of a renderable component. They essentially wrap another render array, which can use a theme hook or a more complex render array to process the data they are responsible for rendering. You can think of them as essentially standardized render arrays.
There are two types of render elements--generic and form input elements. Both have their respective plugin types, annotations and interfaces. They are similar in that they both render a standardized piece of HTML; however, the form input elements have the complexity of having to deal with form processing, ...
