\bibitem is a command used to enter a reference in a thebibliography environment in your document. The syntax for using \bibitem is \bibitem[label]{key}.

The optional [label] is for you to add your own labeling system for the bibliography entry. If no label is set, the entries will be set in numerical order: [1], [2], [3], etc.

The argument {key} is used to reference and link the commands \bibitem and \cite to each other and the information they contain. The command \cite contains the label associated with the intended \bibitem, which is located inside a thebibliography environment, and contains the reference data. Both corresponding \bibitem and \cite must have the same {key}; the easiest way to organize keys is by the author's last name. The secondary braces in the thebibliography environment denote the longest bibliography label you expect to have. So, inserting {foo} means you can have any label shorter or as large as the expression foo. Failure to set this parameter correctly may result in a not so attractive indentation of your bibliography.

The bibliography is a section apart from your main document, and an example of code for the bibliography would look like the following:

\begin{thebibliography}{50}
	\bibitem{Simpson} Homer J. Simpson. \textsl{Mmmmm...donuts}. Evergreen Terrace Printing Co.,
	                  Springfield, SomewhereUSA, 1998
\end{thebibliography}

Then, your main source code would contain the location of the information relating to the \bibitem using \cite. That source code would look similar to this:

My thesis, about the philosophy of The Simpsons\copyright comes from my favorite book \cite{Simpson}.

As it is often difficult to remember the exact citation key once you have many references, Kile provides an easy way to insert a citation. Using LaTeX → References → Cite a list with all the citation keys pops up. Select the correct reference and a citation will be inserted into your document. To update the list of keys, either save the file, or Edit → Refresh Structure, or press F12. With code completion enabled, Kile will show you a list of all the bibitem-labels as soon as you open up a \cite command.

The final result in your document's bibliography would then look like this:

[1] Homer J. Simpson. Mmmmm...donuts. Evergreen Terrace Printing Co., Springfield, SomewhereUSA, 1998.

Kile can also work together with BibT_EX editors, such as KBibT_EX to make it easier to enter citations. When a BibT_EX file is added to the project, Kile will help you complete citation commands, just as described above.