As the name suggests, the bulk of this article will entail the language of the Proto-Indo-Europeans and the descendants of this ancient language.
As a language, Proto-Indo-European(1) is a Bronze Age language still being uncovered by archaeologists and scholars. When referring to Proto-Indo-European, it is assumed the language is unattested and seen as valid. Since the language is prehistoric and largely shrouded by the sands of time, the majority of the current work with the language is guesswork dependent upon developed inferences from descendent languages, much like the hypothetical, reconstructed Gothic language. When referring to the auxiliary Indo-European language, refer to it as Modern Indo-European, or MIE, if you desire. This should distinguish between the constructed language,MIE, and the work of reconstruction with PIE. Both are elicited from Indo-European, but one is rooted in science, and the other is rooted in revivalism.
Indo-European branches include:
[2] http://indo-european.eu/wiki/index.php/Anatolian_languages
[3] http://upload.wikimedia.org/wikipedia/commons/4/4f/IndoEuropeanTree.svg
[4] Wheelock, Frederic M. Wheelock's Latin. New York: HarperCollins. Print. Page xxx.
(1) Proto-Indo-European- called by the short acronym, PIE