Section type widgets are the functions associate with a section in the detail of a record. They are used to manipulate the unstructured data.
A section is the container of a widget, a widget can change the data of the section.
This documentation describes the common functions related to the section type widgets.
When saying add a widget, it actually means add a section with a particular widget. Widget can be added when adding a section. See here for more details.
Default set of widgets can be added directly when creating a new record. See here for more details.
The widget menu is customizable in detail view. Include those commonly used widgets in the menu, can speed up your documentation process. See detail here.