Most Efficient Way of Creating Left Aligned Item/Equation List

I would do this using an itemized list:

If you want the terms left aligned instead, you can use the [align=left, leftmargin=<length>, labelwidth=\widthof{<widest_term>}] options to produce:

Note:

• I used the geometry package and the adjusted the paperwidth= so that the wrapping of the lines can more easily be seen.

Code:

\documentclass{article}
\usepackage[paperwidth=8.0cm,showframe]{geometry}
\usepackage{amsmath}
\usepackage{calc}
\usepackage{enumitem}

\begin{document}
\noindent
The final equation is:
\[ x= (m*n)+P+Q \]
where\par
\begin{itemize}
  \item [$x$] is some really long explanation of this value 
  \item [$n$] is the value of 
  \item [$m$] is the value of
  \item [$P$] is the value of
  \item [$Q$] is the value of
\end{itemize}

\noindent
The final equation is:
\[ x= (m*n)+P+Q \]
where\par
\begin{itemize}[align=left,leftmargin=1.5cm,labelwidth=\widthof{$P+Q$}]
  \item [$x$] is some really long explanation of this value 
  \item [$m*n$] is the their product
  \item [$P+Q$] is the their sum
\end{itemize}
\end{document}

Probably overkill, but I did this for my thesis, it was long ago, when I was a true LaTeX newbie, I didn't even know about memoir

\documentclass[a5paper]{article}

\newenvironment{descr}[1]{\list{}{%
  \setlength{\topsep}{0pt}
  \setlength{\itemsep}{0pt}
  \setlength{\parsep}{0pt}
  \setlength{\itemindent}{0pt}
  \settowidth{\labelwidth}{#1}
  \setlength{\labelsep}{2ex}
  \setlength{\leftmargin}{\parindent}
  \addtolength{\leftmargin}{\labelwidth}
  \addtolength{\leftmargin}{\labelsep}
  }}
  {\endlist}
\newcommand{\itemdesc}[1]{\item[#1\hspace*{\fill}]}

\begin{document}

\begin{descr}{$\vec{f}_{i\alpha j\beta}$}
  \itemdesc{$\vec{R}_i$} es la posición del centro de masas de la molécula $i$, $\vec{R}_i=\frac{1}{M_i}\sum_\alpha m_{i\alpha}\vec{r}_{i\alpha}$.
  \itemdesc{$M_i$} es la masa total de la molécula $i$, $M_i=\sum_\alpha m_{i\alpha}$.
  \itemdesc{$\vec{F}_i$} es la fuerza total en el centro de masas de la molécula $i$, $\vec{F}_i=\sum_{\alpha j\beta}\vec{f}_{i\alpha j\beta}$.
  \itemdesc{$\vec{f}_{i\alpha j\beta}$} es la fuerza existente entre el átomo $\alpha$ de la molécula $i$ y el átomo $\beta$ de la molécula $j$.
  \itemdesc{$\vec{J}_i$} es el tensor momento de inercia de la molécula $i$, $\vec{J}_i=\sum_\alpha m_{i\alpha}(s_{i\alpha}^2\vec{I}-\vec{s}_{i\alpha}\,\vec{s}_{i\alpha}^T)$.
  \itemdesc{$\vec{s}_{i\alpha}$} es el vector posición del átomo $\alpha$ respecto al centro de masas de la molécula $i$, $\vec{s}_{i\alpha}=\vec{r}_{i\alpha}-\vec{R}_i$.
  \itemdesc{$\vec{\omega}_i$} es la velocidad angular de la molécula $i$.
  \itemdesc{$\vec{N}_i$} es el momento de la fuerza total sobre la molécula $i$, $\vec{N}_i=\sum_\alpha\vec{s}_{i\alpha}\times\vec{f}_{i\alpha}$.
\end{descr}

\end{document}

(ignore missing accents)

By contrast, the simple itemize environment (as in Peter Grill's answer) gives you this alignment:

(Note I also changed vertical spacing in my customized environment.)

If your definitions are quite uniform, you can put them in a align environment like Vafa suggested:

\begin{align}
    x &:= (m*n)+P+Q\\
    n &:= ...
\end{align}

But if the left side of the definitions differ to much, it might not look to good anymore. Also it could happen that the right side needs more than one line. In that case you need to break it, which is a sensible topic, compare https://stackoverflow.com/questions/1578127/how-do-i-break-a-long-equation-over-lines. For unnumbered equations you could do

\begin{align*}
    x &:= (m*n)+P+Q+R+S+...\\
    &\quad +Y+Z+\pi\\
    n &:= ...
\end{align*}

Furthermore it is always sensible to give some meaning to the variables you define by putting a descriptive text. Using aligned equations there are two possibilities:

\begin{align}
    x &:= (m*n)+P+Q+R+S+... && \text{number of birds in the cage}\\
    \intertext{where $n$ is the usual number:}
    n &:= ...
\end{align}