These are a handy way to turn any long lens into a macro lens for ultra-close shooting. They are hollow metal tubes that are mounted between the camera and the lens. Typically they come in a set of different lengths which can be combined. It is a good idea to open up the lens a little when using an extension tube, as a little light is lost. It should be noted that they do not work when used with wide lenses.

Glossary of Film Terms - http://homepage.newschool.edu/~schlemoj/film_courses/glossary_of_film_terms/
reprinted with permission