The plasma membrane is an outer boundary and functional "traffic-cop" of every biological cell. Just like the outer layers of multicellular organisms, its primary function is to contain, protect and provide unit structure. The plasma membrane is classified as an organelle of the cell.
The plasma membrane is selectively permeable in that it helps control what moves into and out of its cell (thus the "traffic-cop" label). It consists of a thin structured bilayer of phospholipids and protein molecules; it is found as a component of every biological cell, and serves to divide the inner environment of the cell from the outer environment (much like the skin of animals). The surfaces of plasma membranes are generally studded with proteins that perform different functions, like the reception of nutrients or the anchoring of the flagellum. The plasma membrane is normally about ten nanometers thick.
The structure of the plasma membrane is often referred to as a fluid mosaic because it is a two-dimensional fluid (like a soap bubble) of freely diffusing lipids that is dotted with proteins. These proteins either adhere to the membrane, or span it to provide a bridge from the inside to the outside. Beneath the plasma membrane is the cytoskeleton, which provides anchoring points for integral membrane proteins so that they don't move around too much.
Since the lipid bilayer is semipermeable, only some molecules can pass unhindered through it; these molecules are tiny, like sodium ions, or lipophilic. It is possible for other molecules to pass through the membrane if specific transport molecules carry it.
In bacteria, yeast, and plants, an additional cell wall forms another boundary outside the plasma membrane to provide stiffness and mechanical support.
Web Resources On Plasma Membrane
BeyondBooks: The Plasma Membrane
Cell Structure & Function
Book Resources On Plasma Membrane
Plant Membranes: Endo- And Plasma Membranes of Plant Cells by David G. Robinson
The Plant Plasma Membrane: Structure, Function and Molecular Biology by Larsson et al