He is the father of modern photojournalism. He developed candid or "street" photography which is hugely popular today. He also helped form Magnum Photos a co-op of photographers.
"To take a photograph is to hold one’s breath when all faculties converge in a face of fleeing reality. It is at that moment that mastering an image becomes a great physical and intellectual joy."
I love his portraits the most. The one below is of a young Truman Capote.
This is probably his most popular image.