The /** stuff */
document is also referred to as DocBlock notation.
It's used to document PHP functions, classes, etc.
/**
* A test function
*
* @param foo $bar
* @return baz
*/
function test(foo $bar) { return new baz; }
Some editors make good use of this to perform their Code Insight feature, a very powerful tool that reduces the time you have to spend looking at your old function declarations. Aptana and Zend Studio have this feature, probably others as well.
You can also use it in combination with Reflection to do some sort of AOP, doing runtime inspection of the DocBlock above your declarations. In fact, Doctrine uses it to build an object attribute map for their "Active Record" implementation.