As email is generated on the originating system, and as it travels the internet, the email picks up a record of the path it took in it's journey to your mailbox. If you think of the message as a railcar, you can imagine the internet as the track it travels on. As the car passes through many stations This record is stored in the message header. The header is generally hidden from the user. The system administrator will have to have a copy of this header for him to track down the offender who originated the spam. But on most email readers the user can view the headers easily.
For instance if you are using Microsoft Outlook Express you can right click on the message in the xxxx pane and select properties from the resulting popup menu. This will show you the header. Other email readers usually have a way to display the header, often a SHOW HEADER command in the options menu.