Multi-line equations with explanations on some lines

If you use the amsmath package then this can be done using the align (or align* if you don't want the equations numbered) environment together with the \text command for the text.

\documentclass{minimal}
\usepackage{amsmath}

\begin{document}
\begin{align*}
A &= B \\
&= C && \text{Because of D} \\
&= D && \text{But beware of E} \\
&&& \text{Some more on that}
&= F
\end{align*}
\end{document}

Result (note that the actual result will be centred on the page):

To get the alignment as you have it, you would have:

&A \\
&= B \

at the start.

The amsmath package is loaded by default if you use one of the classes from the AMS, such as amsart.

Loop Space answer works well, however, it requires manually splitting the text. This is not very convenient.

A more convenient way is to use \parbox

Following his/her example:

\documentclass{minimal}
\usepackage{amsmath}

\begin{document}
\begin{align*}
A &= B \\
&= C && \text{Because of D} \\
&= D && \parbox[t]{5cm}{
          Some long description here that will split 
          automatically and then force a fourth line \\ 
          Another line here} \\
&= F
\end{align*}
\end{document}