Inter-component communication in Aura

In practice, a Lightning component’s code cannot directly interact with its parent and vice versa. A parent component cannot manipulate its children or siblings as with standard JavaScript and the DOM. Inter-component communication must be specified by the developer.

Below are the 3 possibilities of communication between components:

Possibility 1: Communicating From Parent component to a child :

Option 1: Attributes 

<!-- parent.cmp-->
<aura:component>
	<c:child name="hello"/>
</aura:component>

Option 2 : aura:methods

Option 3 : Applicaton Event

Possibility 2: Communicating From Child component to parent:

Option 1: Application Events

Option 2: Component events 

Possibility 3: Communication between 2 independent components:

Option 1: Application Events [ provided that they have registered the appropriate event handler.]

To understand the real use cases of the above, it is mandatory to refer to this blog post: ClickHere