1. Persistent - URLs (Uniform Resource Locators, or "Internet addresses") are not persistent and can change quickly. You may be required to actually copy the Internet source cited for inclusion as an appendix or attachment to your paper or speeches. To explain this point, one study followed 64 web pages cited in academic articles over three years. At the conclusion of the study, 48% of the pages could not be accessed. ("URLs: Uniform Resource Locators or Unreliable Resource Locators", Carol Anne Germain, College & Research Libraries, July 2000, pp.359-365). That's why we say check with your professor before citing Internet resources.
2. Consistent - it is possible that the content of the document may not be consistent. It may change over time. In that case, the content of the original document could be completely different from when you first used it. Additonally, beware of "frames." Citing a document in a frame is tricky. A frame is a screen with two or more "windows." Technically, each screen has its own URL. While web browsers can assemble multiple frames on your PC monitor, browsers can only display one address in its location bar no matter how many windows appear -- usually the main page that tells the browser how to assemble it.
Another issue is citing private e-mail letters to help give proper credit. Private e-mails will not be available for "public" viewing to support numbers one and two above in most cases if someone wants to verify your source so those documents may need to be printed. Be sure you first have their permission to be quoted.