Cell Membrane

All cell membranes have the same basic structure. A type of lipid called a phospholipid has a "head" which is hydrophilic, and a "tail" that is hydrophobic.

Image of a bi-polar phospholipid molecule showing the hydrophilic head and hydrophobic tail. Image of the phospholipid bilayer structure of the cell membrane.

They will spontaneously form a double-layer called a bilayer: the hydrophilic heads all face out to the surrounding environment or inwards to the cytoplasm (both based on water). The hydrophobic tails all face each other (the middle of the membrane). Cell membranes are critical for separating the internal cellular environment from the extracellular environment. If the inside and outside of the cell had the same solutions cellular life would not exist.

Proteins either embedded in the membrane or even across the membrane (transmembrane) add other functions to particular cells. These functions may include, transport proteins, cell adhesion molecules, anchors for the cytoskeleton, enzymes, and cell receptors.
Molecules such as cholesterol add stability.
Chemical additions, such as carbohydrates, (the green bits in the image) add identification tags (so your immune system knows it's your cell).

Image of the complex phospholipid bilayer structure of the cell membrane.