How to use <aura:handler/> in Salesforce Lightning?

<aura:handler... > is used for handle standard and custom events.

1. aura:valueInit - :  Indicates that an app or component has been initialized.

<aura:handler name="init" value="{!this}" action="{!c.doInit}"/>

2.aura:waiting -:  Indicates that the app or component is waiting for a response to a server request. This event is fired before aura:doneWaiting.

<aura:handler event="aura:waiting" action="{!c.showSpinner}"/>

3. aura:doneWaiting - :  Indicates that the app or component is done waiting for a response to a server request.

<aura:handler event="aura:doneWaiting" action="{!c.hideSpinner}"/>