The \bibitem command generates an entry labeled by label.
If the label
argument is missing, a number is generated as the label, using
the enumivcounter.
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.