Indenting a whole paragraph

You could use the adjustwidth environment from changepage package- a MWE follows. Note that adjustwidth environment deals with page breaks.

The adjustwidth environment takes 2 arguments: the first is the indent from the left margin, and the second is the indent from the right margin. See the documentation for more details.

\documentclass{report}

\usepackage{lipsum}       % for sample text
\usepackage{changepage}   % for the adjustwidth environment

\begin{document}

\lipsum[1]

\begin{adjustwidth}{2cm}{}
\lipsum[1]
\end{adjustwidth}
\end{document}

If you plan to use this idea a lot, then it's probably worth defining your own environment

\newenvironment{myenv}{\begin{adjustwidth}{2cm}{}}{\end{adjustwidth}}

which could be used as

\begin{myenv}
\lipsum[1]
\end{myenv}

---

Following Werner's comment, you could make your environment take an optional argument that would overwrite the default indentation:

\newenvironment{myenv}[1][2cm]{\begin{adjustwidth}{#1}{}}{\end{adjustwidth}}

Without any additional packages, you could wrap your paragraph in a minipage:

\documentclass{article}
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\begin{document}
\lipsum[1]

\hfill\begin{minipage}{\dimexpr\textwidth-3cm}
\lipsum[2]
\xdef\tpd{\the\prevdepth}
\end{minipage}

\prevdepth\tpd\lipsum[3]
\end{document}
​

In the above example, the paragraph depth correction (via \prevdepth is from How to keep a constant baselineskip when using minipages (or \parboxes)?).

The indent from the left is set to 3cm, but can be modified. Also, if a paragraph indent is required, use \indent within the minipage. All of the afore-mentioned modification can be automated. One caveat is that it will not allow breaking across the page boundary.

---

A rather crude way of doing it as well is to modify \leftskip:

\documentclass{article}
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\begin{document}
\lipsum[1]

\setlength{\leftskip}{3cm}

\lipsum[2]

\setlength{\leftskip}{0pt}

\lipsum[3]
\end{document}
​

Perhaps the easiest way to do that is to use \begin{quote} ... \end{quote} environment. It has a pretty similar effect:

\documentclass{article}
\usepackage{lipsum}

\begin{document}
\lipsum[1]

\begin{quote}
\lipsum[2]
\end{quote}

\lipsum[3]
\end{document}