Microscopic world:

Robert Hooke coined the term cell after observing the dead cork under a lens. He found closely packed empty containers (left over cell walls of the plant) which looked like the cells that monks used to live in a monastery, hence he termed them as cells.

Anton Van Leeuwenhoek observed pond water, tooth scrapings and mouth samples of people who never brushed their teeth and found tiny living moving forms, he called them animalcules. 

Robert Hooke

Anton Van Leeuwenhoek

Robert Hooke and Anton Van Leeuwenhoek, both observed the microorganisms with the help of lenses that are developed to check the quality of clothes. Curiosity is the reason why they wanted to observed different things under the lenses and because of their inquisitiveness a new field of science has born.

General Sizes of cells and other particles:

• A grain of Salt is 0.5 mm.

• A skin cell is 30um (micrometers).

• Red blood cell is 8um.

• A chromosome is 7um.

• HIV is 130nm (nanometers)

• Ribosome is 30nm

Note: 1 millimeter (mm) is 1000 micrometer (um) and 100,000 nm (nanometer).

For more information and better visualization of the above particles please look here.

Micrographia is the book written by Robert Hooke, the scientist who coined the term "cell". When Hooke first observed a wooden cork, he found distinct separations and it looked like monastery cells, hence he termed them as cells.