$$
\begin{array}{c}
\ovalbox{A} \\
\uparrow \downarrow \\
\ovalbox{B} \\
\uparrow \\
\ovalbox{C}
\end{array}
$$
(The fancybox package is used here, so that \ovalbox{B} appears as a capital B in a box with curved corners.)
This is a crude sort of picture of a simple directed graph, and although all I need is something simple like this, I also was to be able to show an arrow pointing from \ovalbox{B} to itself, set to the right of \ovalbox{B}. How can I do that?

pgfmanual.pdf) should contain everything you need. Also see http://tex.stackexchange.com/questions/503/why-is-preferable-to – Caramdir Apr 15 '12 at 19:55