This applies to Netscape prior to version 6.0. From that release onwards, event management follows the guidelines in the DOM level 3 event specification.
On receipt of a call to this method, the receiving object will look at its available set of event handler functions and pass the event to an appropriately mapped handler function. It is essentially an event dispatcher that is granular down to the object level.
The argument value is an Event object that contains information about the event.