Google Drive collaborative documents are made of 4 types of collaborative elements: String, Lists, Maps and Custom objects. A Google Drive collaborative document can have as many of these collaborative elements as they need. As a demonstration, You will find below UI widgets representing collaborative elements of each type.
Collaborative strings are just like normal strings, except that all browser sessions will be notified and updated when the string changes. Events are fired when strings are inserted to the collaborative string or when part of the string is deleted.
Collaborative custom object have properties who's state is shared across sessions. Events are fired when property values are changed.