So if you quote somebody and go through the trouble of listing the book, and then quote him again, you can just say "ibid" instead of relisting the book.<ul><li><a href="http://howto.yahoo.com/ask/19991209.html">Ask Yahoo: "What does Ibid mean?"</a></li></ul>

"ibid" is short for ibidem, Latin for "in the same place". It's an expression used in bibliographies when authors repeatedly cite the same source. So instead of typing out the title of something every time you refer to the book you used in your project, you simply type "Ibid" for each reference after the first one, then cite the page number to which you're referring.

Ibid. is an abbreviation, so it takes a period.
Used only to refer to the work last cited in full. By itself, it means in the last work at the same page.

Id. is the abbreviation for idem (meaning "the same") and is used in citations to refer to the cited authority immediately preceding.

Ibid. or ib. is the abbreviation for ibidem (in the same/very place). Ibidem itself is a combination of "ibi" (in that place) with the demonstrative suffix "dem".