You're missing the mandatory argument to \begin{thebibliography}. In your case it should be
\begin{thebibliography}{9}
(any one digit number is good). Use 99 if you have more than nine items.
It's also better to have a blank line after every item.
See also Argument in "thebibliography" for other information about the problem.
A better way to input your bibliography would be
\begin{thebibliography}{9}
\bibitem {Cluster}
\url{http://dictionary.reference.com/browse/cluster}
\bibitem {install}
\url{https://en.wikibooks.org/wiki/Building_a_Beowulf_Cluster}
\end{thebibliography}
For the \url command you need to load the url package
\usepackage{url}
It is available also if you load hyperref. Note that in the second URL the underscores need not to be escaped.
\begin{thebibliography}. In your case it should be\begin{thebibliography}{9}(any one digit number is good). Use99if you have more than nine items. It's also better to have a blank line after every item. – egreg May 07 '15 at 22:03