Email viruses are--as the name would imply--found in emails. Often they are contained in attachments such as Word documents, which, when opened,allow the virus to spread. They often spread by sending themselves to people in someone's address book after that person has opened the attachment.
One example is the I LOVE YOU virus which, once initialized (the attachment was double-clicked), sent copies of itself to everyone in a person's address book as well as corrupting files on the machine the person opened the attachment on.