Paparazzi is a plural term (paparazzo being the singular form for photographers who take candid photographs of celebrities, usually by relentlessly shadowing them in their public and private activities. Celebrities claiming to have been hounded by such photographers often use "paparazzi" as a pejorative term while news agencies commonly use the word in a broader sense to describe all photographers who take pictures of people of note.

The word paparazzi was popularized after the Federico Fellini 1960 film "La dolce vita". One of the characters in the film is a news photographer named Paparazzo (played by Walter Santesso). In his book Word and Phrase Origins, author Robert Hendrickson writes that Fellini took the name paparazzi from an Italian dialect word for a particularly noisy, buzzing mosquito. In his school days, Fellini remembered a boy who was nicknamed "Paparazzo" (Mosquito), because of his fast talking and constant movements, a name Fellini later applied to the fictional character in "La dolce vita".

Paparazzi sell their work to dozens of magazines and newspapers that publish such photos for their readers and subscribers, and many paparazzi feel that they are helping celebrities and public figures in general by increasing their visibility. Photographers often earn large sums of money for a valuable picture.

Source : http://en.wikipedia.org