What is a non membrane bound organelle?

According to Wikipedia:

The term organelle is the diminutive form of organ meaning 'little tool". Various texts, including the popular Campbell and Reece (6th edition) refer to ribosomes as organelles. Others do not. Some restrict the use to membrane-bound structures. This then leads to some confusion as to how to refer to ribosomes. Consequently many texts (Cormack, Introduction to Histology) delineate between membranous and non-membranous organelles. Non-membranous cell structures, which are sometimes not considered organelles, include: * ribosome * cytoskeleton * flagellum * centriole and microtubule-organizing center (MTOC). So basically the bulletpoints are non-membrane bound organelles.