The documentation just says

To save an object back to the database, call save()

That does not make it clear. Exprimenting, I found that if I include an id, it updates existing entry, while, if I don't, it creates a new row. Does the documentation specify what happens?
Depends on how the Model object was created. If it was queried from the database, UPDATE. If it's a new object and has not been saved before, INSERT.
Does django's `save()` create or update?