Coding an equation with description

Some more flair in managing the "where" clause, and formatting of the conditions:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\usepackage{array}% http://ctan.org/pkg/array
\begin{document}

\begin{gather}
  P_{xi}=\overline{U}_x+\sigma_x
         \frac{\sum_k^{Nu}D_{kx}\times\left(\frac{S_{ki}-\overline{U}_k}{\sigma_k}\right)}
              {\sum_k^{Nu}D_{kx}},
\intertext{Where:}
  \begin{tabular}{>{$}r<{$}@{\ :\ }l}
    P_{xi} & is the predicted rate for user~$x$ on item~$i$ \\
    S_{ki} & is the rate of song~$i$ given by user~$k$ \\
    D_{kx} & the correlation between user~$x$ and user~$k$ \\
    \overline{U}_x & the average rate over user~$x$ \\
    \overline{U}_k & the average rate over user~$k$ \\
    \sigma_x & is the standard deviation of all the rates of user~$x$
  \end{tabular}\nonumber
\end{gather}

\end{document}​

Even though your question was thoroughly answered here's my solution, it differs only in not using a table but a list:

\documentclass{article}

\usepackage{amsmath}
\usepackage{amssymb}

\usepackage{enumitem}

\begin{document}

\[ P_{xi}=\overline{U_{x}}+\sigma_{x}\frac{\sum^{Nu}_{k}D_{kx}\times 
    \left( \frac{S_{ki}-\overline{U_{k}}}{\sigma_{k}}\right)}{\sum^{Nu}_{k}D_{kx}} \]
Where:
\begin{itemize}[label=]
    \item $P_{xi}$: is the predicted rate for user $x$ on item $i$
    \item $S_{ki}$: is the rate of song $i$ given by user $k$
    \item $\overline{U_{x}}$: is the average rate of user $x$
    \item $\overline{U_{k}}$: is the average rate of user $k$
    \item $\sigma_{x}$: is the standard deviation of all the rates of user $x$
\end{itemize}

\end{document}

Which produces the following result:

You can add the line over the variables using \overline. The text below can be done e.g. using a tabular.

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{gather}
    P_{xi} = \overline{U_{x}} + \sigma_{x} \frac
    {\sum_k^{Nu} D_{kx} \times \left( \frac {(S_{ki}-\overline{U_k})}{\sigma_k} \right) }
    {\sum_k^{Nu} D_{kx}},
\end{gather}
Where:\\
\hspace*{3em}
\begin{tabular}{rl}
    $P_{xi}$:& is the predicted rate for user $x$ on item $i$. \\
    $S_{ki}$:& is the rate of song $i$ given by user $k$. \\
    ... & ... \\
\end{tabular}

\end{document}

You can change the space between the columns in the tabular by adding @{\hspace{<your space>}} between the r and l column type specifier.