Reasons for a Query to be Non-Updatable

One subject we are frequently getting questions on, and which we really need a link for, is about why some queries are updatable and some are not. Most of us have some understanding of the issue, but putting all the reasons down clearly as a reference point is something that Microsoft have already done.

Where better, therefore, to look for a reference on this than MSDN (When can I update data from a query?)