Vocab

Buffer: a text buffer. Doesn't have to correspond to an on-disk text file, can also be in-memory scratch buffers.
Editor: an open view into a text buffer.
EditorState: the view position and cursors/selections of an editor for a particular buffer.
Mark: an abstract representation of a range in the text. Used for representing cursors and selections, among other things.
Change: a record of simultaneous edits to a buffer. Includes data to both apply and undo the edits.
History: a history of changes made to a buffer. Used for undo.