Comparing prokaryotic and Eukaryotic cells (Biological Principles)Watch
Unicellular organisms are organisms which are one cell. In this one cell, it has everything it needs to survive. E.g. Amoeba, paramecium...
Multicellular organisms are organisms that have multiple cells with different functions to keep the organism alive. These cells don't merge together necessarily, they group up to make tissue etc...(explained below) E.g. Human beings, fish, plants....
I think you may be confused with the fact that:
1)a group of the same cells is called tissue
2) a group of tissue is called an organ
3) a group of organs make a system
4) a group of systems make an organism (simplified)
Hope this makes sense. Let me know if you need some more help.