The first systematic theory of the relationships between human languages began when Sir William Jones proposed in 1788 that Greek and Latin, the classical languages of Europe, and Sanskrit, the classical language of India, had all descended from a common source. The evidence for this came from both the structure of the languages -- Sanskrit grammar has similarities to Greek and to nothing else -- and the vobcabulary of the languages. Thus, "father" in English compares to "Vater" in German, "pater" in Latin, "patêr" in Greek, "pitr." in Sanskrit, "pedar" in Persian, etc. On the other hand, "father" in Arabic is "ab," which hardly seems like any of the others. This became the theory of Indo-European languages, and today the hypothetical language that would be the common source for all Indo-European languages is called Proto-Indo-European.
Source
This incomplete site is devoted to information on ancient Indo-European languages spoken in the Balkans: Thracian, Dacian and Moesian. It also contains a corpus of Greek and Latin sources, as well as information on ancient Balkans geography, and possible soltdm.tripod.com
Links to various projects involving the Indo-European language, maintained by Dr. Deborah W. Anderson, Dept. of Linguistics, UC Berkeley. www.indo-european.org
Comparative presentation of the numerals 1-10 in the Indo-European languages, including reconstructions in Proto-Indo-European and various intermediate proto (reconstructed) languages. members.tripod.com/~rjschellen/IENums.htm
Collection of scholarly material devoted to Indo-European linguistics, from the Institute of Comparative Linguistics of the Johann Wolfgang Goethe-Universität, Frankfurt am Main, and related institutions [Multilingual site, incl. English-language materia titus.uni-frankfurt.de/indexe.htm
An interesting hodge-podge of pages on various branches and individual member languages of the Indo-European family. There are vocabularies of "lesser-known" languages, and links to major on-line dictionaries of better-known ones. www.wordgumbo.com/ie/cmp