Hi Tauseef,

Application Adapters are Hook Plugins that unlike our standard hooks which affect the entire portal, will allow us to scope the changes to a Site.

They were introduced in Liferay 6.1 and for the moment they only support jsp overwriting.

To create a Application Adapter, all you need is a Hook with custom JSPs and "custom-jsp-global" set to false.

<custom-jsp-dir>/META-INF/custom_jsps</custom-jsp-dir>
<custom-jsp-global>false</custom-jsp-global>

For more information, you can refer this wiki:

https://www.liferay.com/community/wiki/-/wiki/Main/Application+Adapters

Thanks:
Ankit