Robert Hooke is considered by many one of the greatest scientists and inventors of all time. His invention of the compound microscope changed science in his era, and he used it to change the field of science observation forever. In 1655, he published a book on his findings using the compound microscope called Micrographia, which was a collection of his observations and detailed drawings of plants and insects he had observed. It was in this book and these drawings that Hooke's most famous discovery was published.
His greatest claim in the book, and what he is most famous to many for, was the discovery of plant cells. Hooke observed a piece of cork under his microscope and realized it looked like a "honey-comb" and was made of irregular "pores, or cells". He later went on to say that these "microscoplial pores" were the first that either he or anyone else had observed. He also went on to observe similar "cells" in wood and other plants, confirming his findings. He was even called upon later by the Royal Society to confirm the work of another scientist, Antony Van Leeuwenhoek, who discovered bacteria and protozoa. Hooke's agreement with Leeuwenhoek's findings helped to push science further ahead by confirming these results.
He also made important contributions in other fields. For example, "Hooke's law" in physics is still used today to describe elasticity in a spring. As an inventor, he also invented one of the first forms of a respirator, and a spring that allowed clocks to function more effectively. Hooke also was one of the earliest paleontologists who realized fossils showed a record of animals that used to live on earth, and that they could represent extinct species that no longer exist. Overall, Robert Hooke was a very interesting man who made important contributions to many fields of science throughout his life. Hope this helps!