Henri Cartier-Bresson is one of the most famous photographers of all time. His images, taken during a period when photography was still a rare thing, provide us with incredible insights into ordinary life during the first half of the 20th century.
Another more recently famous photographer is Vivian Maier who took pictures on the streets of Chicago in the mid-20th century. What both Cartier-Bresson and Maier have in common is that they were street photographers. They took candid pictures of people in the street, usually without their subject's knowledge. In doing so they created bodies of work that provide us with lasting insights into society in their times.
Street photography has a history as long as the history of the camera. However, in more recent times it is increasingly frowned upon.