While LaTeX (or any other TeX-derived package) isn't really like a compiler, people regularly want to do compiler-like things using it. Common requirements are conditional 'compilation' and 'block comments', and several LaTeX-specific means to this end are available.
The simple \newcommand{\gobble}[1]{} and
\iffalse ... \fi aren't really satisfactory (as a general
solution) for comments, since the matter being skipped is nevertheless
scanned by TeX, not always as you would expect. The scanning
imposes restrictions on what you're allowed to skip; this may not be a
problem in today's job, but could return to bite you tomorrow.
For an example of surprises that may come to bite you, consider the
following example (derived from real user experience):
The\iffalse % ignoring this bit consider what happens if we use \verb|\iftrue| -- a surprise \fi
\iftrue is spotted by TeX as it scans, ignoring the
\verb command; so the \iffalse isn't terminated by the
following \fi. Also, \gobble is pretty inefficient for any
but trivial arguments, since all the matter to be skipped is copied to
the argument stack before being ignored.
If your requirement is for a document from which whole chapters (or
the like) are missing, consider the LaTeX
\include/\includeonly system. If you '\include' your
files (rather than \input them - see
What's going on in my \include commands?),
LaTeX writes macro traces of what's going on at the end of each
chapter to the .aux file; by using \includeonly, you can give
LaTeX an exhaustive list of the files that are needed. Files that
don't get \included are skipped entirely, but the document
processing continues as if they were there, and page, footnote,
and other numbers are not disturbed. Note that you can choose which
sections you want included interactively, using the
askinclude package.
The inverse can be done using the excludeonly package: this
allows you to exclude a (list of) \included files from your
document, by means of an \excludeonly command.
If you want to select particular pages of your document, use Heiko Oberdiek's pagesel or the selectp packages. You can do something similar with an existing PDF document (which you may have compiled using pdflatex in the first place), using the pdfpages package. The job is then done with a document looking like:
\documentclass{article}
\usepackage[final]{pdfpages}
\begin{document}
\includepdf[pages=30-40]{yoursource.pdf}
\end{document}
(To include all of the document, you write
\includepdf[pages=-]{yoursource.pdf}
omitting the start and end pages in the optional argument.)
If you want flexible facilities for including or excluding small portions of a file, consider the comment, version or optional packages.
The comment package allows you to declare areas of a document to be
included or excluded; you make these declarations in the preamble of
your file. The command \includecomment{version-name}
declares an environment version-name whose content will
be included in your document, while
\excludecomment{version-name} defines an environment whose
content will be excluded from the document. The package uses a method
for exclusion that is pretty robust, and can cope with ill-formed
bunches of text (e.g., with unbalanced braces or \if commands).
These FAQs employ the comment package to alter layout
between the printed (two-column) version and the PDF version
for browsing; there are narrowversion and
wideversion for the two versions of the file.
version offers similar facilities to comment.sty
(i.e., \includeversion and \excludeversion commands);
it's far "lighter weight", but is less robust (and in particular,
cannot deal with very large areas of text being included/excluded).
A significant development of version, confusingly called
versions (i.e., merely a plural of the old package name).
Versions adds a command
\markversion{version-name} which defines an environment
that prints the included text, with a clear printed mark around it.
optional defines a command \opt; its first argument is
an 'inclusion flag', and its second is text to be included or
excluded. Text to be included or excluded must be well-formed
(nothing mismatched), and should not be too big - if a large body of
text is needed, \input should be used in the argument.
The documentation (in the package file itself) tells you
how to declare which sections are to be included: this can be done in
the document preamble, but the documentation also suggests ways in
which it can be done on the command line that invokes LaTeX, or
interactively.
And, not least of this style of conditional compilation,
verbatim (which should be available in any distribution)
defines a comment environment, which enables the
dedicated user of the source text editor to suppress bits of a
LaTeX source file. The memoir class offers the same
environment.
An interesting variation is the xcomment package. This defines an environment whose body is all excluded, apart from environments named in its argument. So, for example:
\begin{xcomment}{figure,table}
This text is not included
\begin{figure}
This figure is included
\end{figure}
This is not included, either
\begin{table}
This table also included
\end{table}
...
\end{xcomment}
A further valuable twist is offered by the extract package. This allows you to produce a "partial copy" of an existing document: the package was developed to permit production of a "book of examples" from a set of lecture notes. The package documentation shows the following usage:
\usepackage[
active,
generate=foobar,
extract-env={figure,table},
extract-cmd={chapter,section}
]{extract}
which will cause the package to produce a file foobar.tex
containing all the figure and table
environments, and the \chapter and \section commands, from
the document being processed. The new file foobar.tex is
generated in the course of an otherwise ordinary run on the 'master'
document. The package provides a good number of other facilities,
including (numeric or labelled) ranges of environments to extract, and
an extract environment which you can use to create complete
ready-to-run LaTeX documents with stuff you've extracted.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=conditional