Service
Exchange Online Archiving Service Description (TechNet)
http://technet.microsoft.com/en-us/library/exchange-online-archiving-service-description.aspx
Exchange Online Archiving (Website)
Understanding
Understanding Exchange Online Archiving
http://technet.microsoft.com/en-us/library/hh529934(v=exchg.141).aspx
Understanding Personal Archives
http://technet.microsoft.com/en-us/library/dd979795(v=exchg.141).aspx
Configuration and Requirements
Configure Exchange Online Archiving
http://technet.microsoft.com/en-us/library/hh529915(v=exchg.141).aspx
License requirements for Personal Archive and retention policies