wikipedia on candid photography

"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."
Candid photography is also set off against the voyeuristic stalking involved in animal photography, sports photography or photographic journalistic intrusion, which all have a very strong technical focus on getting distant objects photographed, e.g. by using telephoto lenses. Candid photography's setup includes a photographer who is typically there with the "subjects" to be photographed if not close, and not hidden. People photographed on candid shots either ignore or accept the close presence of the photographer's camera without posing for photos.
The events documented are often private, they involve people in close relation to something they do, or they involve people's relation to each other. Candids are the kinds of pictures taken at children's birthday parties and on Christmas morning, opening the presents; the pictures a wedding photographer takes at the reception, of people dancing, eating, and socializing with other guests. They are taken at leisure, or at special occasions, they show people as they are when they do not prepare to be photographed."

For event photography the pressmen are after the un-posed,unplanned and immediate photographs. Spontaneous but not necessarily unobtrusive. Theoretically this involves certain problems regarding an understanding of 'reality' however in practice one tries to capture a story which one is an observer of as well as a particpant.