It's commonly necessary to present text in two languages 'together' on a page, or on a two-page spread. For this to be satisfactory, one usually needs some sort of alignment between the two texts.
The parallel package satisfies the need, permitting typesetting in two columns (not necessarily of the same width) on one page, or on the two opposing pages of a two-page spread.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=parallel