Understanding IIdentity and IPrincipal

So i've read MSDN a couple of times but it doesnt really help me fully understand this clearly. I've provided a link to the Interfaces that im referring to 
IIdentity - http://msdn.microsoft.com/en-us/library/system.security.principal.iidentity.isauthenticated(v=vs.110).aspx IPrincipal - http://msdn.microsoft.com/en-us/library/system.security.principal.iprincipal(v=vs.110).aspx
1. Are both used with ASP .Net membership only and not the new ASP .Net Identity? 2. If i create a project which uses forms authentication and need to write my own classes would i then use the above Interfaces and implement them in order to get to the logged in user/finding the user logged onto the site?
Thanks