Component

aura:component
1. it is a functional unit of aura which can contain
i. html
ii. standard or custom
2. file extension *.cmp
3. the bundle contains below elements

Component Type Purpose
Component XML
Component Controller Client side controller
Helper Javascript      Client side controller (common set of js)
Style CSS    Style code and Classes
Documentation    Documentation
Renderer Javascript 

Renderer
SVG Icon 

Icon element

4. Attributes
1. access : scope of the component
public : default one used to expose the component within namespace only.
global : can be used within namespace and outside the namespace

2. controller : to refer an apex class

3. description : to include the comment

4. extends : specifying the extending the application
eg.,
<aura:component extends=”namespace:componentName”>

5. extensible : it is a boolean value which says your current application can be extendable or nor
eg.,
<aura:component extensible=”true”>

6. template : specifiy the name of the custom template

7. implements : used to specify the list of interface

Navigation
Setup -> Developer Console -> File -> New -> Lightning Component.
Example 1
Name : Comp_one.cmp

<aura:component >
    <h1>Ameerpet</h1>
    <h1>Contact: 9969696699</h1>
</aura:component>

Include the above component into an application like below. This is mandatory to preview the component. A component can not be previewed.

<aura:application >
    <h1>SFDC Lightning Training</h1>
    <c:Comp_one/>
</aura:application>

Example 2
StudentPersonal.cmp

<aura:component >
   <h1>Student Name: Ranjith Krishnan</h1>
    <h1>Age: 30</h1>
    <h1>Phone: 9840409993</h1>
</aura:component>

StudentAddress.cmp

<aura:component >
    <h1>Anna Nagar</h1>
    <h1>Chennai</h1>
    <h1>Tamil Nadu</h1>    
    <br/>
    <c:StudentCourse/>
</aura:component>

Preview the component using application

<aura:application >
    <c:StudentPersonal/>
    <br/>
    <c:StudentAddress/>
</aura:application>

Components can also be nested

StudentCourse.cmp

<aura:component >
   <h2>Salesforce Lightning Course</h2>
</aura:component>

StudentAddress.cmp

<aura:component >
    <h1>Anna Nagar</h1>
    <h1>Chennai</h1>
    <h1>Tamil Nadu</h1>    
    <br/>
    <c:StudentCourse/>
</aura:component>