\begin{thebibliography}{widest-label}
\bibitem[label]{cite_key}
.
.
.
\end{thebibliography}
The thebibliography environment produces a bibliography or reference list. In the article style, this reference list is labeled "References"; in the report style, it is labeled "Bibliography".
\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 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.
\cite[text]{key_list}
The key_list argument is a list of citation keys. This command generates an in-text citation to the references associated with the keys in key_list by entries on the aux file read by the
\begin{document} command.
\nocite{key_list}
The \nocite command produces no text, but writes key_list, which is a list of one or more citation keys, on the aux file.