It's some sort of flow chart. Its graphical design ensures that a proper structure will be kept, opposite to the common flow charts which will allow users to generate "spaghetti code". Thus Nassi Shneiderman diagrams are used particularly in the turbo pascal and delphi world for didactical reasons.
Here are some links:
http://cui.unige.ch/eao/grammed/doc/diplome.html#sec4.3
http://www.preview.org/cgi-bin/nav_e.pl?t=scshot21.htm
http://www.google.com/search?q=nassi...uche&hl=en&lr=