Amongst Van Leeuwenhoek's many discoveries are: in 1674 he discovered infusoria (protists in modern zo÷logical classification), in 1676 he discovered bacteria, in 1677 he discovered spermatozoa and in 1682 he discovered the banded pattern of muscular fibers ("Biography of Antony...").
He is best known for his work on the improvement of the microscope and for his contributions towards the establishment of microbiology. Using his handcrafted microscopes he was the first to observe and describe single celled organisms, which he originally referred to as animalcules, and which we now refer to as microorganisms ("Antony van Leeuwenhoek").
How does this affect me?
Leeuwenhoek was the first to observe single-celled organisms that later scientists have applied towards the discovery of multiple-celled organisms so without this prior knowledge, the existence of both single and multiple-celled organisms probably would not be known today.
(image from http://en.wikivisual.com/images/5/5e/Anton_van_Leeuwenhoek.png)