Hooke developed the compound microscope and described the chambers of cork as "cells".
Leeuwenhook developed very fine compound microscopes and looked at just about everything he could think of. He observed a whole lot of things (bacteria, protozoa, blood cells, sperm cells....), their sizes, their movements, but didn't really try to figure out exactly what he was seeing.
Schleiden concluded that cells are the basic unit of plants.
Schwann concluded that cells are the basic unit of animals.
Virchow proclaimed that cells can only come from pre-existing cells.