\bibitem[label]{cite_key}
The \bibitem command generates an entry labeled by label. If the label argument is missing, a number is generated as the label, using the enumiv counter. The cite_key is a mnemonic consisting of any sequence of letters, numbers, and punctuation symbols not containing a comma.
\bibitem and \cite are used together much as \label and \ref to produce a correspondence between items in the Reference or Bibliography (thebibliography environment).
This command writes an entry on the aux file containing cite_key and the item's label. When this aux file is read by the \begin{document} command, the item's label is associated with cite_key, causing the reference to cite_key by a \cite command to produce the associated label.