Indo-European is the hypothesized ancestor of most of the languages that are spoken in Europe and some that are spoken in present day India and Iran. By way of the comparative method we are able to reconstruct from the oldest languages of the Indo-European daughter branches its sound and grammar. This reconstruction informs us about etymology, linguistic genealogy, historical sound change and prehistoric society.
Germanic is the language family to which English, Dutch, German, Frisian and the Scandinavian languages belong. It belongs to the Indo-European language family and distinguishes itself from its linguistic neigbours Celtic, Slavic and Italic. By using the oldest Germanic languages we can reconstruct the hypothetic ancestor language of all Germanic languages, which we call Proto-Germanic. Apart from sharing the same language, the Germanic peoples in Late Antiquity also seem to have shared some cultural elements, such as a common pantheon. Unfortunately, the nationalist discourse of the early twentieth century wherein Germanic identity was heralded as superior and the following second world war made many notions about Germanic society suspect in serious academia, even up to the present day.
Despite being associated with fairies, art and role-playing, the study of Celtic is an old and established discipline in academia. Celtic was once spoken from Turkey in the east to the British Isles in the West. It is preserved in inscriptions from Europe from Antiquity and in the medieval literatures of the Celtic peoples on the British isles. Celtic languages survive only on the west most shores of Europe, namely Brittany, Wales, Ireland, Manx and Scotland. These languages on the British Isles seem to have undergone a similar restructuring which make them very unlike other Indo-European languages. Gaulish, a continental Celtic language stills looks a lot like Old-Latin and Greek. Contrary to what the comics of Asterix would have you believe the Gauls were a sophisticated people who had an urban civilization in the South of Gaul where literacy in their own language was common, though not very pervasive.
Romance is the ancestor of the Romance languages such as French, Spanish, Italian and Rumanian. Often Latin is named as the ancestor of these languages, but Romance is not entirely identical to Latin. Latin is the standardized literary language of Rome, especially around the year 0 AD. Romance continues the spoken variety of Latin and evolved a lot from the language people learn in high school.