A biological membrane or biomembrane is a membrane which acts as a barrier within or around a cell. It is a lipid bilayer, being composed of a double layer of lipid-class molecules, specifically phospholipids, with occasional proteins intertwined, some of which function as channels.
Such membranes typically define enclosed spaces or compartments in which cells may maintain a chemical or biochemical environment that differs from the outside. For example, the membrane around peroxisomes shields the rest of the cell from peroxides, and the plasma membrane separates a cell from its surrounding medium. Most organelles are defined by such membranes.
Probably the most important feature of a biomembrane is that it is a selectively permeable structure. This means that the size, charge and other chemical properties of the atoms and molecules attempting to cross it will determine whether they succeed to do so. Selective permeability is essential for effective separation of a cell or organelle from its surroundings.
If a particle is too large or otherwise unable to cross the membrane by itself, but is still needed by a cell, it could either go through one of the protein channels, or be taken in by means of endocytosis.
More general information on this topic may be found in the article at cell membrane.