The cytoplasm is everything in the cell that is not an organelle. It contains many of the proteins, molecules, and important substances that the cell needs, as well as keeping the organelles separate. It is similar to air for us; it has many essential ingredients for continued existence; it provides a medium in which the organelles can exist (living in a vacuum is not very easy), and pretty much fills up space.

For more information about cells and cytology, go to the Virtual Cell, or check out the many other sites on cytoplasm.