Explain core topic extension, extension module, extension template in Hybris? | Hybris interview question for fresher

Hello Friends,
Today I choose very basic topic of hybris and it is extension, I know that you all are aware about extension but let’s read again.

What is extension?
Basically extension is a unit of functionality in hybris. In other work you can say each extension is used for a specific functionality in hybris. If you look the architecture of Hybris then you can found that if you need to customize any functionality of Hybris then you do by creating your own extension so every extension in hybris contains
1. Business logic
2. Type definition (items.xml)
3. Web application
4. hmc configuration
Next question is What is extension module ?
In my last Hybris interview I got question that what is extension and extension module. So basically extension module is building block or you can say structural element of extension
lets take example of yacceleratorstorefront, where if you look in their extensioninfo.xml file then you found
 

<webmodule jspcompile="false" webroot="/yacceleratorstorefront"/>

Let’s take an example of yacceleratorcore, where if you look in their extenioninfo.xml file then you found
 

<coremodule generated="true" manager="de.hybris.platform.yacceleratorcore.jalo.YAcceleratorCoreManager" packageroot="de.hybris.platform.yacceleratorcore"/>

<hmcmodule extensionclassname="de.hybris.platform.yacceleratorcore.hmc.YAcceleratorCoreHMCExtension"/>

so all above webmodule, coremodule, hmcmodule is called extension module and each module has offers a certain set of functionality
below details of functionality of each module given above
Coremodule -Declarations and localizations of types, Jalo item classes (items.xml)
Webmodule - Web application
Hmcmodule - hybris Management Console (hMC) configuration and localization for it, custom hybris Management Console elements, such as editors, tabs, display chips.
 

Now my final question is what is extension template?

I think you already know about extgen - is ant command to create extension. Basically for new extension you are choose extension template like yempty etc so that yempty is extension template.