Single-celled organisms are not efficient as all functions in 1 cell. In multi-cellular they are all adapted, they become specialised but all have same genes. Certain genes are switched on (expressed) depending on cell. Shape and number of organelles vary.
Tissues - are a collection of similar cells performing specific functions. Cells aggregated together to work efficiently. Epithelial tissues are in animals and are a sheet of tissues lining organ surface with a protective/secretory function.
Organs - Combo of tissues co-ordinated to perform a variety of functions. tissues aggregated into organs. Veins and arteries are organs but capillaries are not.
Organ Systems - system grouped for more efficiency of particular functions. Organs work as 1 unit.