It is important to use these abbreviations literally and to punctuate them correctly. Many writers confuse "e.g." and "i.e.," and many type "et al." improperly or do not properly recognize what words it represents.
The abbreviation "e.g." is from the Latin exempli gratia and means, literally, "for example." Periods come after each letter and a comma normally follows unless the example is a single word and no pause is natural:
Any facial response (e.g., a surprised blink of both eyes) was recorded.
The abbreviation "i.e." is from the Latin id est, meaning "that is." Loosely, "i.e." is used to mean "therefore" or "in other words." Periods come after each letter and a comma normally follows, depending on whether the wording following the abbreviation dictates a natural pause:
In every case Angle 1 was greater than Angle 2—i.e., every viewer perceived a circle.
The phrase "et al."—from the Latin et alii, which literally means "and others"—must always be typed with a space between the two words and with a period after the "l" (since the "al." is an abbreviation). A comma does not follow the abbreviation unless the sentence’s grammar requires it. Some journals italicize the phrase because it comes from the Latin, but most do not.
Schweiger et al. applied the neural network method.
Never begin a sentence with any of these three abbreviations; if you want to begin a sentence with "for example" or "therefore," always write the words out.
For an entertaining look at how "et al." is used, visit this site: