Tissue:

* Definition: A group of similar cells that work together to perform a specific function.

* Examples: Muscle tissue (contracts to allow movement), nervous tissue (transmits signals), epithelial tissue (covers surfaces), connective tissue (provides support and structure).

* Level of Organization: The basic level of organization within an organism, coming after cells and before organs.

Organ System:

* Definition: A group of organs that work together to perform a major bodily function.

* Examples: Digestive system (breaks down food), circulatory system (transports blood), respiratory system (exchanges gases), nervous system (controls and coordinates body functions).

* Level of Organization: A higher level of organization than tissues, involving multiple organs working in coordination.

Here's a simple analogy:

* Imagine a house. The bricks are like cells, the walls are like tissues (made of many bricks), and the entire house is like an organ system (the walls, roof, plumbing, etc., all working together).

Key Differences:

* Complexity: Organ systems are more complex than tissues, consisting of multiple different tissues working in concert.

* Function: Tissues have a specific function, while organ systems perform broader, more complex bodily functions.

* Structure: Tissues are made of similar cells, while organs are made of multiple tissue types.

In summary: Tissues are the building blocks of organs, and organ systems are formed when multiple organs work together to carry out major bodily functions.