Answer :

Micro-organisms are the organisms which are so small in size that we cannot see them with naked eyes. Eg- bacteria, fungi, algae etc.
The study of micro-organisms is called microbiology. They were discovered in 1674 by Antonie Leeuwenhoek

They are organisms which are so small that they cannot be seen by the naked eye. They need to be seen by a microscope. For eg- Amoeba, etc.