Etymology is the study of the origin of words. Some words have been derived from other languages, possibly in a changed form. Through old texts and comparisons with other languages, etymologists try to reconstruct the history of words - when did they enter a language, from what source, and how did their form and meaning change?
Etymologists also try to reconstruct information about languages that are too old for any direct information (such as written texts) to be known. By comparing words in related languages, one can learn about their shared parent language. This way, word roots have been found which can be traced all the way back to the origin of the Indo-European language family.
See also: Proto-language