The most common types of malware are Trojans, viruses and worms, which in turn can be programmed for a wide variety of intentions.
Unlike Trojans, viruses and worms can spread themselves, and in most cases they are designed to damage or destroy files. The difference between viruses and worms is that worms can execute themselves, whereas viruses need the user to do so. Worms usually spread via a network, e.g. via email programs, through which they could mass-phish and thus spread further in the emails. Viruses attach themselves to files and are executed every time the user runs the programme in question. The most typical types of viruses are boot sector viruses, file viruses, macro viruses (they often appear in phishing) and script viruses.