Are queries run completely before a Cursor can be used?

The documentation for Cursors at http://www.postgresql.org/docs/9.2/static/plpgsql-cursors.html says that:

"Rather than executing a whole query at once, it is possible to set up a cursor that encapsulates the query, and then read the query result a few rows at a time. One reason for doing this is to avoid memory overrun when the result contains a large number of rows"