Hello Satheesh
Here is the details you need.

Component events: to talk to a parent using the capture and bubbling mechanism, like with DOM events. Usually, one component is interested by the event, like an event aggregator.

Application events: to broadcast to other components and not exclusively ancestors. Applications events can talk to many components that can be interested by the event. The broadcast can be boxed to an area of the DOM (everything below a DIV for example).

About $A.getEvt() & cmp.getEvent():
Events have been seriously revamped, and the distinction between Component and Application events has been reduced. However, this rework has not yet been translated into an updated API.
I need to remove references to the $A.getEvt() and cmp.getEvent() API because we are still working on a new version of the API. My apologies for the confusion.
About value providers:
Lightning has various value providers: "v" for attributes, "c" for controller, and "e" for events.
Application Event API:
Those are obtained from the global (hence $A.get()) event value provider:
            var evt = $A.get("e.myNamespace:myEvent");

Component Event API:
Component must reference events by name, much like an aura:id, and retrieve them from its component (hence cmp.get()) value provider:
            var evt = cmp.get("e.myEvent");

You would then declare myEvent on the component as:
            <aura:registerEvent name="myEvent" type="namespace:eventName"/>

Why dow have this API? Declaring events on a component allows other components to call controller methods. You had to declare a handler:
            <aura:handler name="myEvent" action="{!c.myEventHandler}"/>

That allowed you to call myEventHandler from another component and respect the interface:
            cmp.get("e.myEvent").fire();

Hope this will help you.
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Thanks