Information Retrieval for Chemists: 84:260 

• ibid. means "in the same place" used frequently so that the in the second, third or fourth reference to the same journal or book title, one does not have to repeat identical information. ONLY the information which is different will be given in the second reference. See samples. Use only when several references in a row refer to the same journal.
• idem means "the same" used when the name of the author is the same as the previous reference.  You may occasionally see idem, ibid. meaning that the article referred to has the same author as the previous article, and comes from the same journal.
• et al. means "and others"  This is used instead of writing out all the authors of a particular article.  As a general rule of thumb, if you have 3 authors or less, write out their names.  If you have four or more, cite the first author's name in full and use et al.  

• Example:  Bennington, P.H., Stow, W.F.,   Steiger, R.J., Brown, J.T., Smith, D.K.  would be cited as:

• loc. cit. means "in the place cited"
• op. cit.  means  "in the work cited"

 ibid. references, numbers 22, 25, 29-31
Idem, ibid. references, numbers 35-36

If the references were written out, they would look like this:

22. L.F.Fieser and X.A. Dominguez, Am.Soc., 75, 1704 (1953); L.F. Fieser and W.Y. Huang, Am.Soc., 75, 4837, (1953).
25. P.A. Levene, Organ.Syn. Coll.Vol., 2, 320,(1943)
29. O.Kamm, Organ.Syn. Coll. Vol., 1, 445, (1941)
30. F. C. Whitmore and F.H. Hamilton, Organ. Syn Coll. Vol., 1, 492, (1941)
31. W.E. Kuhn, Organ.Syn. Coll. Vol.,  2, 447, (1943)
35. R. Kuhn and A. Winterstein, Ber., 65, 1737, (1932)
36. R. Kuhn and A. Winterstein, Ber., 65, 1742 (1932)