\bibitem[label]{cite_key}
The \bibitem command generates an entry labelled by label.
If the label argument is missing, a number is generated as the
label, using the enumi counter. The cite_key is
any sequence of letters, numbers, and punctuation symbols not containing
a comma. 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.