What is the Fluid Mosaic Model?
A model for the dynamics in cellular membranes
The model describes how cellular membranes i.e. lipid bilayers are organized. Despite the fluidity, lipid bilayers can form certain domains with different characterisrics and compositions. The cell can use different mixtures of lipids to create a 'mosaic' or 'patchwork' of domains.
An example of such domains are so called 'lipid rafts' which are aggregates of certain lipids (mostly cholesterol and sphingomyelins). In these rafts the lateral diffusion of membrane-bound proteins is strongly reduced, thereby forming stable complexes to facilitate, for example, signal-processing and transduction.
Note that a lot about how or why a cell creates these domains is still unknown.