The cell membrane composed of major lipids like phospholipid and proteins in a bilayer . The lipids are arranged such that polar head towards outer side which is hydrophilic and non polar tails towards the inner side which is hydrophobic. Hence non polar end is protected from aqueous end and also phospholipid layer comprises of cholesterol. The peripheral protein lies on the surface of membrane and integral protein are totally or partially burried in the membrane.
The widely accepted model is fluid mosaic model. According to this quasi fluid nature of lipids enable lateral movement of proteins within overall bilayer. This ability to move within the membrane is measured as fluidity.