Defining vs Non-defining relative clauses

Exercises
http://www.edufind.com/ENGLISH/GRAMMAR/REL2.CFM
http://www.ego4u.com/en/cram-up/grammar/relative-clauses/exercises?12http://ies1libertas.edu.gva.es/departamentos/ingles/PDFs/primerob/erelcla1b1.pdf
http://docweb.khk.be/Jen%20Van%20Loock/ex5relativepronouns.htm

http://seulaliaingles.iespana.es/new%20exercises%20for%20the%20web/relative_clauses.htm
Grammar explanation
http://www.ego4u.com/en/cram-up/grammar/relative-clauses/exercises?04
http://perso.wanadoo.es/autoenglish/gr.nonrelat.i.htm