When using e-mail, you can send and receive messages containing file attachments. Attached files can range from documents and photos to music and movies. Just about any type of file can be attached to an e-mail message, including some harmful ones. It is important to use caution when opening e-mail attachments, especially those received from unfamiliar senders, as they may contain viruses or other malicious code.
Computer viruses are programs that execute and replicate themselves without a computer user's knowledge. Viruses can seriously damage your operating system or hardware or they can be simply annoying, launching unwanted pop-up advertisements as you browse the Web.
A worm can spread through e-mail very quickly and may even be received from someone you know. A worm typically infects your e-mail client, spreading into your contact list. The worm then sends messages to your friends, co-workers and others. These messages are infected with copies of the worm, which then spreads to other computers without any of the email recipients being aware of it.
Just like the Trojan Horse of Greek legend, Trojans are viruses which are disguised as harmless programs, such as antivirus software. The Trojan infects your computer when you open and use the program.
"Web bugs" are bits of code hidden in images which are contained in an otherwise ordinary e-mail (usually junk mail from unfamiliar senders). The hidden code lets spammers know that your e-mail address is valid, which in turn causes them to send you more spam or sell your e-mail address to other spammers. Web bugs can also make your system's domain name and IP address known to the perpetrator of the bug.