The important feature of a phospholipid that makes it well suited for a cell membrane is its amphipathic nature. That means that one part of the molecule (the phosphate part) is very polar, so it mixes well with water, and the other part (the lipid tail of the molecule) is hydrophobic because it is non-polar. So these molecules naturally form bilayers in which the tails point together, and the heads of the molecules face either the cytoplasm or the extracellular environment. This forms a barrier, allowing the cell to maintain different concentrations of molecules inside the cell compared with outside the cell.
