Viruses

The words get used interchangeably and they describe different behaviours.

A virus attaches itself to something else and needs you to run that thing. A worm needs nobody: it spreads by itself from machine to machine, which is why worms have historically caused the fastest, largest outbreaks. A Trojan hides inside something you wanted, so you install it deliberately. The distinction matters because it tells you how something spreads, and how it spreads determines whether the answer is about user behaviour, network design or what you allow to be installed.