The Answer:

Thanks for e-mailing Information Please. "i.e." and "e.g." are abbreviations of Latin phrases and each has its own specific meaning.

"i.e." stands for id est, meaning "that is." It is used to explain what exactly something means, or to say the same thing in other words. What comes before "i.e." equals what comes after. For example:

Mom took the family pet—i.e., Rover—to the veterinarian for his annual check-up.

In this case, Rover is the only family pet and we use "i.e." to provide more information about the same term.

"e.g." stands for exempli gratia, meaning "for the sake of example." It is used to provide an example of one type of the thing that came before. For example:

Veterinarians care for lots of animals, e.g., dogs, cats, and guinea pigs.

Always use a comma after either of these abbreviations.

—The Fact Monster