http://grammar.about.com/od/words/a/latinabbrev.htm
These days, a sound rule for using Latin abbreviations (such as e.g., etc., et al., and i.e.) is not to use them at all.
Such abbreviations were popular when Latin was the universal academic language in Europe and America. That's no longer the case. Because so few people study Latin anymore, expressions that once were common have fallen into disuse or misuse.
In our time, Latin abbreviations are generally appropriate only in special circumstances that prize brevity, as in footnotes, bibliographies, and technical lists. But if we must use Latin abbreviations, we should learn how to use them correctly.
Let's look at four Latin abbreviations that still appear in modern English--and that are often confused with one another.
1) e.g. (for example)
Example:"One and the same thing can at the same time be good, bad, and indifferent, e.g., music is good to the melancholy, bad to those who mourn, and neither good nor bad to the deaf."
(Baruch Spinoza)
What e.g. stands for in Latin: exempli gratia
What e.g. means in English: for example
How e.g. is punctuated: with periods after e and g, followed by a comma
How e.g. is used: to introduce examples
How e.g. should not be used: as a synonym for etc. or to introduce an all-inclusive list.
How e.g. can be avoided: use "for example" or "for instance" instead.
2) etc. (and so on)
Example:"None of my own experiences ever finds its way into my work. However, the stages of my life--motherhood, middle age,etc.--often influence my subject matter."
(Anne Tyler)
What etc. stands for in Latin: et cetera
What etc. means in English: and other things
How etc. is punctuated: with a period at the end
How etc. is used: in informal or technical writing, to suggest the logical continuation of a list of things (not, as a general rule, of people)
How etc. should not be used: (1) after and; (2) as a synonym for e.g. or et al.; (3) in reference to people; (4) vaguely to refer to "other things" that are not at all clear to the reader.
How etc. can be avoided: specify all of the items in a list or use "and so on."
3) et al. (and others)
Example:"Why is it that any time any of us mentions that women can be something other than just mothers, teachers, nurses, et al., some mother, teacher, nurse, et al. comes in demanding that we re-affirm that it’s okay to be a mother, teacher, nurse, et al.?"
(Shelley Powers)
What et al. stands for in Latin: et alii
What et al. means in English: and others
How et al. is punctuated: with a period after the l but not after the t
How et al. is used: in bibliographic citations or in informal or technical writing to suggest the logical continuation of a list of people (not things)
How et al. should not be used: (1) after and; (2) as a synonym for e.g.or etc.; (3) in reference to things; (4) vaguely to refer to "others" that are not at all clear to the reader.
How et al. can be avoided: specify all of the items in a list or use "and so on."
4) i.e. (that is)
Example:"Software is like entropy. It is difficult to grasp, weighs nothing, and obeys the second law of thermodynamics; i.e., it always increases."
(Norman R. Augustine)
What i.e. stands for in Latin: id est
What i.e. means in English: that is
How i.e. is punctuated: with periods after i and e, followed by a comma
How i.e. is used: to introduce an explanatory phrase or clause
How i.e. should not be used: as a synonym for because.
How i.e. can be avoided: use "that is" instead.