Various web browser security problems, Y2K software problems. Usually an email that gets mailed in chain letter fashion describing some devastating highly unlikely type of virus, you can usually spot a hoax because there's no file attachment, no reference to a third party who can validate the claim and the general 'tone' of the message. A program that neither replicates or copies itself, but does damage or compromises the security of the computer. Typically it relies on someone emailing it to you, it does not email itself, it may arrive in the form of a joke program or software of some sort. Virus A program or code that replicates, that is infects another program, boot sector, partition sector or document that supports macros by inserting itself or attaching itself to that medium.
Most viruses just replicate, a lot also do damage.
Worm A program that makes copies of itself, for example from one disk drive to another, or by copying itself using email or some other transport mechanism. It may do damage and compromise the security of the computer. It may arrive in the form of a joke program or software of some sort. This payload might cause the computer to crash or to behave in an unexpected fashion. This payload might attempt to gain access to passwords or other system-level security settings.