From the documentation:
“When an event does not have any associated data, the class raising the event uses System.EventHandler as the delegate and System.EventArgs for the event data. Events that have associated data use classes that derive from EventArgs …”
The EventArg class is used by events that do not pass additional information to an event handler when an event is raised.
“The sender object indicates what raised the event.”
The ‘sender’ identifies the object that raised the event (the textobox that was modified in this case). This could be useful if you have more than one object using the same event handler.