One often needs clear indications of how a document has changed, but the commonest technique, "change bars" (also known as "revision bars"), requires surprisingly much trickery of the programmer (the problem being that TeX 'proper' doesn't provide the programmer with any information about the "current position" from which a putative start- or end-point of a bar might be calculated; PDFTeX does provide the information, but we're not aware yet of any programmer taking advantage of the fact to write a PDFTeX-based changebar package).
The simplest package that offers change bars is Peter Schmitt's backgrnd.tex; this was written as a Plain TeX application that patches the output routine, but it appears to work at least on simple LaTeX documents. Wise LaTeX users will be alerted by the information that backgrnd patches their output routine, and will watch its behaviour very carefully (patching the LaTeX output routine is not something to undertake lightly...).
The longest-established solution is the changebar package,
which uses \special commands supplied by the driver you're using.
You need therefore to tell the package which driver to generate
\specials for (in the same way that you need to tell the
graphics package); the list of available drivers is pretty
restricted, but does include dvips. The package comes with
a shell script chbar.sh (for use on Unix machines) that
will compare two documents and generate a third which is marked-up
with changebar macros to highlight changes. The (excellent)
shareware WinEDT editor has a macro that will generate
changebar (or other) macros to show differences from an
earlier version of your file, stored in an
RCS-controlled repository - see
http://www.winedt.org/Macros/LaTeX/RCSdiff.php
The vertbars package uses the techniques of the
lineno package (which must be present); it's thus the
smallest of the packages for change bar marking, since it leaves all
the trickery to another package. The framed package is
another that provides bars as a side-effect of other desirable
functionality: its leftbar environment is simply a
stripped-down frame (note, though, that the environment makes a
separate paragraph of its contents, so it is best used when the
convention is to mark a whole changed paragraph.
Finally, the memoir class allows marginal editorial comments, which you can obviously use to delimit areas of changed text.
Another way to keep track of changes is employed by some word-processors - to produce a document that embodies both "old" and "new" versions. The Perl script latexdiff does this for LaTeX documents; you feed it the two documents, and it produces a new LaTeX document in which the changes are very visible. An example of the output is embedded in the documentation, latexdiff-man.pdf (part of the distribution). A rudimentary revision facilility is provided by another Perl script, latexrevise, which accepts or rejects all changes. Manual editing of the difference file can be used to accept or reject selected changes only.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=changebars