In Exchange 2010, you can place a mailbox on retention hold or legal hold. Both holds serve a different purpose. It’s important to understand the functionality provided by both.
Retention Hold: As the Messaging Records Management page in the EMC suggests (see Figure 1), retention hold is used to halt retention policy, which means the Managed Folder Assistant (MFA) does not move or delete items from the mailbox. It’s typically used when a user may not have mailbox access for an extended duration, for example, when the user’s on vacation. You can also use retention hold during the initial phase of MRM deployment and remove it as users become more comfortable with MRM functionality.
Read more at source: http://blogs.technet.com/b/exchange/archive/2011/08/16/retention-hold-and-litigation-hold-in-exchange-2010.aspx