The phospholipid bilayer is the membrane which surrounds all living cells. The two main components of this membrane are phospholipids (giving the membrane its name) and proteins. Together, these determine how the membrane behaves.
A phospholipid has three distinct parts: Glycerol and Phosphate, which form the head; and a Fatty Acid tail. The heads also contain nitrate groups. These heads are hydrophilic, which means that they are attracted to water. By contrast the tails are hydrophobic - they don't like water. Because of this, the phospholipids form a bilayer, with the hydrophobic tails grouping together in the centre, away from water, while the hydrophilic heads are attracted to the water on the outside, thus forming a protective barrier for the tails. It is called a bilayer because it is a layer of two, with one layer of phospholipid heads facing the outside of the cell, and another facing the inside. The phospholipids are not forced too closely together, which means that the membrane is not rigid, which is what gives the cell the 'wobbly' effect we are taught in our early years of education. This means that the membrane has a certain degree of flexibility.
The bilayer is highly selective about what it lets through, as its primary function is as a protective barrier for the cell. Certain substances must be allowed in and out for the cell (and the surrounding cells) to survive and function, while others must be kept out. For example, water passes through the bilayer by osmosis, following the concentration gradient, that is, moving from where there is lots of water, to where there is less. This means that water passes both in and out of the cell, depending on the gradient, keeping the water levels fairly constant. Osmosis is known as passive transport, because it requires no input of energy to happen. In essence, it does it by itself. Other small particles also pass through the membrane in this manner.
Other substances however, are either too large to simply diffuse through the membrane, or are charged (they are ions), and so are repelled by the phospholipids. This is where our proteins come in. There are two basic types of protein embedded in the cell membrane: Channel and Carrier. Channel proteins facilitate passive transport, by simply providing a channel through which certain ions (such as hydrogenH+), which cannot diffuse through the bilayer, may flow. These ions also flow down a concentration gradient, which is why this is also passive transport.
However, not everything follows along a concentration gradient, such potassium (K+), which must be constantly taken up into the cell, against the gradient, in order for the cell to function. For molecules such as these, the membrane has carrier proteins, which are very specific (each protein 'carries' a particular molecule), and only allow the passage of one or two molecules at a time. These proteins facilitate active transport, which requires energy input, becasue it is going against the diffusion gradient. Certain carrier proteins also facilitate passive transport, such as that for glucose.
Together, these phospholipids and proteins maintain the internal environment of the cell, only allowing in what should be in, and protecting the cell from what could harm it. Failure of this membrane to do so often results in cell death, and in many cases disease for the person concerned. As such, it is a highly complex, and vital structure.