Cell Organisation

Cell: They are the basic building blocks of the universe which make up all living organisms. They can either be prokaryotic(bacterias) or eukaryotic(Organisms with nucleus). There are also specialised cells which carry out specific function and their structure helps them do that. For example there are red blood cells.
Tissue: A group of similiar cells that work together to carry out a specific function. For example, mammals have muscular, glandular and epithelial tissues that all work to carry out specific functions.
Organ: A group of different tissues that work together to carry out a specific function. For example, there's the liver which works to produce bile.
Organ System: A group of organs working together to carry out a specific function. For example there is the digestion system.