Server-side Synchronization (SSS) is an alternative to Outlook synchronization which requires the CRM for Outlook add-in running in your Outlook for syncronization to take place.
To help you understand SSS we have recently published a new whitepaper “Server-side Synchronization in Dynamics CRM and Dynamics CRM Online”. The whitepaper describes in detail the process for synchronizing accounts, contacts, tasks, and email between Dynamics CRM and Exchange, which is handled by the Asynchronous Processing Service.
Get it here