A parbox is a box whose contents are created in paragraph mode. The \parbox has two mandatory arguments:
A \parbox command is used for a parbox containing a small piece of text, with nothing fancy inside. In particular, you shouldn't use any of the paragraph-making environments inside a \parbox argument. For larger pieces of text, including ones containing a paragraph-making environment, you should use a minipage environment.
The \parbox command is fragile.