Short Answer 
The cell is the basic unit of life, with types categorized as prokaryotic or eukaryotic. Cells group together to form tissues, which then combine to create organs, ultimately leading to the formation of an organism through the integration of various systems and functions.
Step 1: Understanding the Basic Unit – Cell
The cell is the fundamental unit of all living organisms. It serves as the essential building block, responsible for carrying out vital functions necessary for life. Cells can perform various tasks depending on their type and are classified as prokaryotic or eukaryotic, with distinct structures and functionalities.
Step 2: Grouping Cells into Tissues
When similar types of cells come together, they form a tissue. Tissues are characterized by their shared structure and function, working collectively to perform specific tasks in an organism. There are several types of tissues such as epithelial, connective, muscle, and nervous tissues, each serving unique roles.
Step 3: Building Complexity with Organs and Systems
Multiple tissues combine to create an organ, which performs more complex functions necessary for survival. These organs are organized into systems, where groups of organs coordinate their efforts to carry out broader functions in a living organism. For example, the heart (organ) works with blood vessels in the circulatory system. Finally, when all systems are integrated, they form an organism, a complete living entity.