Unicellular Organism:
Unicellular organisms are living organisms that consist of a single cell. These organisms are known as the simplest form of life and include various types of bacteria, archaea, protozoa, and algae. The cell of a unicellular organism is capable of performing all the necessary life functions, such as obtaining nutrients, reproducing, and eliminating waste.

- Structure: Unicellular organisms have a simple structure. They are microscopic in size and their body is made up of a single cell. This cell carries out all the necessary functions for the organism to survive.

- Reproduction: Unicellular organisms reproduce through various methods, such as binary fission, budding, and spore formation. In binary fission, the cell divides into two identical daughter cells. In budding, a small bud grows on the parent cell and eventually detaches to become a new organism. Spore formation involves the production of specialized cells that can grow into new organisms under favorable conditions.

- Metabolism: Unicellular organisms have a relatively simple metabolism. They obtain nutrients from their environment through processes like diffusion or active transport. These nutrients are then used to carry out cellular respiration, which provides energy for the organism. Waste products are eliminated through processes like diffusion or exocytosis.

Multicellular Organism:
Multicellular organisms are living organisms that are composed of more than one cell. These organisms range from simple multicellular organisms like sponges to complex ones like humans. The cells in a multicellular organism are specialized and organized into tissues, organs, and systems to perform specific functions.

- Structure: Multicellular organisms have a complex structure. They are made up of multiple cells that are organized into different tissues, organs, and systems. Each cell has a specific function and works together with other cells to carry out the overall functions of the organism.

- Reproduction: Multicellular organisms reproduce through sexual or asexual reproduction. Sexual reproduction involves the fusion of gametes from two parents, resulting in offspring with genetic variation. Asexual reproduction involves the production of offspring from a single parent, without the involvement of gametes.

- Metabolism: Multicellular organisms have a more complex metabolism compared to unicellular organisms. They have specialized organs and systems that allow for the efficient uptake of nutrients and elimination of waste. These organisms require more energy to sustain their larger size and more complex functions.

In conclusion, the main difference between unicellular and multicellular organisms lies in their structure, reproduction, and metabolism. Unicellular organisms are composed of a single cell, reproduce asexually, and have a simple metabolism. On the other hand, multicellular organisms are made up of multiple cells, reproduce sexually or asexually, and have a more complex metabolism.