Candid photography is snapshot photography that focuses on spontaneity rather than technique, on perfecting the immersion of a camera within events rather than focusing on setting up a staged situation, focusing on lengthy camera setup, or focusing on particularly strong lenses.
The photographic setup of candid photography is best described as un-posed and unplanned, immediate and unobtrusive. This is in contrast to classic photography, which includes aspects such as carefully staged portrait photography, landscape photography or object photography.
Candid photography is supposed to catch rare instances of life from the very immersion into it, rather than to produce imagery of still life, to catch rare moments of "reality" which presupposes a definition of "reality."
The above taken from Wikipedia