Is there a circle symbol?

It's \circ:

\documentclass{article}
\usepackage{fixltx2e} % To make \( ... \) robust
\begin{document}
\( f \circ f \)
\end{document}

Probably this is a good chance to recall the Detexify website, where you can simply draw the symbol you want, and obtain the needed code.

I'm a bad illustrator, but for me, after drawing the circle, \circ was the first hit.