A membrane is a thin soft layer of material. A membrane separates two things.
A membrane can mean a thin layer of cells or tissue. This layer covers the body or an organ, or separates, or lines a body cavity. An example is the mucous membrane that is the skin that lines the inside of your nose and mouth. An epithelial membrane has two parts, one part is epithelial tissue and the other is connective tissue.
Membranes of cells
There are many different types of membranes in a cell. The cell membrane, also called the plasma membrane, covers one cell. Membranes also divide the cell into different spaces called organelles. Organelles are special areas of the cell that do different work. For example, the nucleus holds the DNA in a cell. The mitochondria make energy for the cell.
Membranes in cells are made of lipids (fats) and protein. The lipids keep the inside of the cell or the organelle separate from the outside. The proteins do many things. Plasma membranes give the cell messages from outside. They let some things (like glucose, calcium, and potassium) go into and out of the cell.