Activating directory synchronization should be considered a long-term commitment. After you have activated directory synchronization, you can edit only synchronized objects by using on-premises apps.
When you install the Directory Synchronization tool, the Directory Synchronization Configuration wizard creates a service account to read from your local Active Directory and write to the Office 365 synchronization database. The wizard creates this account using both your local Active Directory permissions and your Office 365 permissions, which you provide as part of setup.
When you activate directory synchronization, you are turning on this feature for your Office 365 subscription. You must activate directory synchronization before you install the Directory Synchronization tool.
To run the Directory Synchronization tool, you must have administrator permissions for the following:
· The computer running the Directory Synchronization tool.
· Your company’s local Active Directory Credentials.
· Your company’s Office 365 account administrator Credentials.
Link to install Directory synchronization tool - http://onlinehelp.microsoft.com/en-us/Office365-enterprises/ff652545.aspx
Link to get prepared for AD Sync - http://onlinehelp.microsoft.com/en-us/Office365-enterprises/ff652544.aspx