IN 1988, a Cornell graduate student, Robert Tappan Morris, let loose a computer worm on the fledgling version of the Internet. He said it was meant to be an experiment, but the code he wrote spun out of his control, affecting roughly 50,000 computers connected to the network at the time. Mr. Morris, who happened to be the son of a National Security Agency scientist, became one of the earliest convicted hackers.

Pranksters followed, scrawling their signatures on Web sites for bragging rights. Organized crime found it could extract millions of dollars by hacking into banks. Then, just as the rest of us had gotten used to living much of our lives online, came the hacktivists.

Calling themselves Anonymous, they hacked into conference calls among agents of the F.B.I. (in January) and broke into the computer networks of the Vatican (last week). It has become impossible to tell who and what Anonymous might target — or exactly who within Anonymous might be behind it. Critics and defenders argue endlessly about how much of what they do should be treated as political protest — or strictly as a crime.

Leaderless, multinational and known by the ubiquitous, sly Guy Fawkes mask, Anonymous is fueled by a raft of causes, from repression in Tunisia to animal rights in Tennessee to a defense of the whistle-blower site WikiLeaks.