Examples include: Not all of these are found in the same type of cell. Most prokaryotes lack organelles (by definition, they lack a nucleus). Ignore alex.tan, he's a medic :P

As for what an organelle is I suspect that being membrane bound is the best definition. I disagree with Gritchka; the ribosome is a cellular machine, and was never an independant entity. I guess it's not too important anyway.

See: Endosymbiotic Hypothesis