The cell membrane separates the organelles and cytoplasm from the exterior of the cell. The membrane also allows in certain materials to ions and organic molecules.
http://www.biology-online.org/dictionary/Cell_membrane
http://www.johnkyrk.com/cellmembrane.html
http://www.biology-online.org/dictionary/Cell_membrane
http://www.johnkyrk.com/cellmembrane.html