Service
Exchange Online Archiving Service Description (TechNet) https://technet.microsoft.com/en-us/library/exchange-online-archiving-service-description.aspx
Exchange Online Archiving (Website) https://office.microsoft.com/en-us/exchange/microsoft-exchange-online-archiving-archiving-email-FX103763589.aspx
Understanding
Understanding Exchange Online Archiving https://technet.microsoft.com/en-us/library/hh529934(v=exchg.141).aspx
Understanding Personal Archives https://technet.microsoft.com/en-us/library/dd979795(v=exchg.141).aspx
Configuration and Requirements
Configure Exchange Online Archiving https://technet.microsoft.com/en-us/library/hh529915(v=exchg.141).aspx
License requirements for Personal Archive and retention policies https://office.microsoft.com/en-001/outlook-help/license-requirements-for-personal-archive-and-retention-policies-HA102576659.aspx