Organisms that are composed of many cells working together to form tissues, organs, and organ systems are referred to as

Eukaryotes. 

There are two main division of organisms in the wide spectrum of species. Eukaryotic organisms and prokaryotic organisms. 
These organisms is divided based on the cell number they have. Further, eukaryotes have a dynamic function and strata in terms of cell structure and I'm not discussing in its internal parts but the external varieties invovled. 
Eukaryotes are unison of different cells that function specifically however, interdependent of one another. That is why they are called cells, and group of cells are called tissues and so on.

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