I.e. is an abbreviation for the Latin phrase id est meaning "that is," or "in other words." The term introduces an explanation of a previous paraphrase, as in: "Dr. Doolittle, reported that he had only 'a few' pets, i.e., he had more animals on his property than the London Zoo." I.e. is only capitalized at the beginning of a sentence. The periods and closing comma are used in American English, though they're unnecessary in English speaking-countries outside of North America, i.e., the UK, Australia, and Ireland.