LyX is a frontend for LaTeX under X11. It combines the comfortable use of a word processor with the high quality of LaTeX type setting. Documents are almost WYSIWYG-like displayed. Low level attributes ("large italic") are not directly chosen, but high level layouts for each paragraph. Of course, low level formatting is still possible.
Using the LaTeX compiler, the LyX document is translated into a .dvi file. The reverse process, to process an already existing LaTeX document with the LyX editor, is not possible (except as a plain ASCII file).