If you compare microorganisms with a human being, our organs would be like their organelles. They are small organs of the cell, which have different functions. For example, eukaryotes have mitochondria, which is known as the power plant of the cell and provides the cell with energy, or chloroplasts, which enable plants and algae to do photosynthesis. Another important cell organelle is the ribosome, in which proteins with specific functions are produced.
Explanation for children:
Many living beings consist of many cells. These cells again consist of many small building blocks. One example of such building blocks are organelles. They are similar to our organs, each organelle has its own function. Some organelles are the energy suppliers for the cell, they are called mitochondria. Other organelles are important for photosynthesis, they are called chloroplasts. Furthermore there are organelles in which, like in a factory, compounds are produced that are essential for us to survive, they are called ribosomes.