When you receive email in HTML format (that is, as a web page), the message will usually contain links to images hosted on a remote server. When you display those images, the sender of the message will know that you've read his message, when you read it, who your ISP is, and approximately where you are. You may regard this as an invasion of your privacy. If the message is spam, then your address has been confirmed as valid and you've been confirmed as someone who opens spam messages. That will ensure you get even more spam.
If you're using the Mail application, you can prevent that information from being sent out.
Select Mail ▹ Preferences ▹ Viewing from the menu bar and uncheck the box marked Display remote images in HTML messages. You'll still be able to display images when you want to by clicking the button marked Load Images in the top right corner of the message window.