The cell membrane is a fluid of mosaic lipids and proteins. Being fluid indicates that the membrane "flows like water", and mosaic means that it is heterogenous in its appearance.
Phospholipid
Contains a phosphate group in its head
is an amphipathic molecule meaning that it has a hydrophilic head that gets attracted by water and a hydrophobic tail which gets repelled.
Saturation
The higher the saturation, the less space there is for phospholipids to move, thus making the cell membrane less fluid.
Unsaturated Phospholipids
Introduce kinks, which free up some space in the membrane making the cell membrane more fluid.
Presence of Cholesterol
Effect of cholesterol on fluidity depends on temperature.
At low temperatures, cholesterol prevents phospholipids from packing closely , thus increasing fluidity.
At high temperatures, cholesterol prevents them from scattering away, thus decreasing fluidity.
