It's easiest just to give an example:

\begin{thebibliography}{[AHU]} \bibitem[AHU]{ahu} Aho, A.,\ Hopcroft, J.,\ and Ullman, J.\ (1976). {\em{The Design and Analysis of Computer Algorithms.}} Addison Wesley, Reading, Mass. \bibitem[AT]{AT} Auslander, L. and Tolmieri, R. (1979). Is Computing with the Fast Fourier Transform Pure or Applied Mathematics? {\em{Bulletin (New Series) of the AMS Vol. 1}} 847-897. \end{thebibliography}yields

We have first specified the largest bibliographic label
with (`{AHU}`

),and then
specified that the bibliography entries are to be
labelled `[AHU]`

and `[AT]`

. If you prefer a
numbered bibliography, you can leave out the bracketed argument.
(The commands here would then begin `\bibitem{ahu}`

`Aho,...`

and
`\bibitem{at} Auslander,...`

.) In that case, would number
the bibliography entries automatically. Also, there is a
command `\cite{ }`

for making references in the text to a
bibliography item which works a lot like `\ref{ }`

(described above). You can type `\cite{`

*key*`}`

where
*key* appears in curly braces to a
`\bibitem`

command; will then insert the correct
reference number in brackets.
For example, if we had let number the bibliography above,
we could type

It was proved in \cite{ahu}, ...to produce ``It was proved in [AHU], ...''.