return false from within a jQuery event handler is effectively the same as calling both
e.stopPropagation on the passed jQuery.Event object.
e.preventDefault() will prevent the default event from occuring,
e.stopPropagation() will prevent the event from bubbling up and
return false will do both. Note that this behaviour differs from normal (non-jQuery) event handlers, in which, notably,
return false does not stop the event from bubbling up.
Source: John Resig