@Salesforce Identity : Can you please explain the differnece between two user licenses ' Identity' and 'External Identity'? If someone can provide an example, that would be great. 

I have a specific scenario which I am trying to resolve. Our organization stores all internal users credentials in Active directory. They want to build a intranet portal which can give seamless access to other cloud based applications. User would need to login to this portal using AD creds and then access other apps using SSO.

Can this intranet portal be Salesforce community where we add tiles for other applications?  How can we implement this sceanrio?

Thanks.