What is the diference between a Roster Group and Multi-User Chat Rooms.

I'am new to XMPP protocol and I'am working on a chat application in android using smack library. I'am stuck in creating group chat. I want to create a group, add members to it and send message to all members of a group (like whatsApp group). When searching about group chat i got two things

1. Group : creating group using createGroup(String name) method present in Roster class.
2. Multi User chat : creating a Multi-user chat using MultiUserChatManager .

I'am getting confused regarding what is Group and what is Multi-User chat? Can anyone explain the basic difference between them and which one should i use and also how to send message to everyone in group ?Smack Dev