The cell membrane is a lipid bilayer that surrounds the entire cell. It is a lipid bilayer in the form of a continuous sheet. The nuclear membrane is not a continuous sheet, but rather is made up of a series of vesicles that come together to form the envelope around the nucleus. The vesicles themselves are lipid bilayers like the cell membrane, but they are "flattened" to form the nuclear membrane. Then of course, the functions of the two membranes are different.
This is a simplified answer to the question. Much more can be said about the differences between the two membranes.