It's often necessary to typeset part of a document in landscape orientation; to achieve this, one needs not only to change the page dimensions, but also to instruct the output device to print the strange page differently.
There are two "ordinary" mechanisms for doing two slight variations of landscape typesetting:
sidewaysfigure and
sidewaystable environments which create floats that
occupy a whole page.
tabbing
environment, or a huge table typeset using longtable or
supertabular), use the lscape package (or
pdflscape if you're generating PDF output, whether
using PDFLaTeX or dvips and generating PDF from
that). Both packages define an environment landscape, which
clears the current page and restarts typesetting in landscape
orientation (and clears the page at the end of the environment
before returning to portrait orientation).
To set an entire document in landscape orientation, one might use
lscape around the whole document. A better option is the
landscape option of the geometry package; if you
also give it dvips or pdftex option,
geometry also emits the rotation instructions to cause the
output to be properly oriented. The memoir class has the same
facilities, in this respect, as does geometry.
A word of warning: most current TeX previewers do not honour
rotation requests in .dvi files (the exception is
the fpTeX previewer WinDVI). If your previewer is not
capable of rotation, your best bet is to convert your output to PostScript
or to PDF, and to view these 'final' forms with an appropriate
viewer.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=landscape