Method in class:
Method in class, namespaced and preferred version:
Indicates a function or method in a class to call. See more information at the USER cObject.
Depending on implementation the class or function name (but not the method name) should probably be prefixed with "user_". The prefix can be changed in the $TYPO3_CONF_VARS config though. The function / method is normally called with 2 parameters, $conf (TS configuration) and $content (some content to be processed and returned).
Also if you call a method in a class, it is checked (when using the USER/USER_INT objects) whether a class with the same name, but prefixed with "ux_" is present and if so, this class is instantiated instead. See the document "Inside TYPO3" for more information on extending classes in TYPO3!