The terms cell, tissue, organ, organ system, and organism represent increasing levels of biological organization.
Cell: The basic structural and functional unit of all living organisms. It is the smallest unit capable of carrying out all life processes. Cells can be prokaryotic (lacking a nucleus) or eukaryotic (possessing a nucleus and other membrane-bound organelles).
Tissue: A group of similar cells that perform a specific function. For example, epithelial tissue covers surfaces, connective tissue supports and connects, muscle tissue allows for movement, and nervous tissue transmits signals.
Organ: A structure composed of two or more different tissues that work together to perform a specific function. Examples include the heart (made of muscle and connective tissue), the stomach (made of epithelial, connective, muscle, and nervous tissue), and the lungs (made of epithelial and connective tissue).
Organ System: A group of organs that work together to perform a major function in the body. Examples include the digestive system (stomach, intestines, etc.), the circulatory system (heart, blood vessels, etc.), and the nervous system (brain, spinal cord, etc.).
Organism: A complete, individual living thing capable of carrying out all life processes. An organism is the highest level of organization.
Relationship: These levels are hierarchical. Cells form tissues, tissues form organs, organs form organ systems, and organ systems make up an organism. Each level builds upon the previous one, with increasing complexity and specialization. For example, muscle cells (cells) combine to form muscle tissue, which forms the heart (organ), which is part of the circulatory system (organ system), which is part of a living organism.