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). This
defines 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.
A word of warning: most current TeX previewers do not honour
rotation requests in .dvi files (the exceptions are the
(commercial) Y&Y previewer
dviwindo,
and 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.