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. The scanning imposes restrictions one what you're
allowed to skip; this may not be a problem in today's job, but
could return to bite you tomorrow. Furthermore, \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.
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.
comment allows you to declare areas of a document to be
included or excluded; you make these declarations in the preamble of
your file. Its exclusion method is pretty robust, and can cope with
ill-formed bunches of text (e.g., with unbalanced braces or \if
commands).
version offers similar facilities to comment.sty; it's far "lighter weight", but is less robust (and in particular, cannot deal with very large areas of text being included/excluded).
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.
Finally, 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.