Spam: the practice of sending unwanted e-mail messages, in large quantities to an indiscriminate set of recipients. These do not necessarily cause damage but are time consuming, use network resources and may be offensive.
Virus: a computer program that can copy itself and infect a computer (typically to do damage) without the permission or knowledge of the user. Usually sent in attachments to emails.
Phishing: the use of a fake website (normally pretending to be a legitimate one) that directs users to enter sensitive details - names, addresses, bank & credit card details etc – so this information can be used for criminally fraudulent purposes. Links to such sites can be part of a spam message.
Trojan: a program that appears to perform a useful function to hide the fact it is providing unauthorised access to the PC that could allow third parties to control the device, save files to it or even watch activity. Can be easily and unwittingly download.
Worm: a self-replicating computer program that uses a network to send copies of itself to other computers on the network. May not cause same level of damage as a virus but – at the very least - will use up system resources, thereby slowing performance.