Google provides an application called Google Apps Sync for Microsoft Outlook to enable you to use Outlook to access your Google email, contact lists, and calendar.
With Google Apps Sync for Microsoft Outlook®, your mail, contacts, and calendar events are stored in your Google account on the web and in Outlook on your computer. Google Apps Sync makes sure data is the same in both places by regularly copying, or synchronizing it back and forth. Incoming messages, meeting invitations, and contacts are downloaded from the web to Outlook, while changes you make locally sync back up with the web. Because it's stored on the web, you can access this information from any computer anywhere—either from Outlook or by logging in to Google Apps with your web browser.
Some Outlook data, however, doesn't get synchronized with Google Apps. This includes tasks, notes, journal entries, and other features that Google Apps doesn't support. You can still use these features in Outlook, say, to track tasks on your To Do list. But the tasks don't synchronize with Google Apps and so won't be available when you sign in to your Google Apps account.
The Outlook Feature Comparison list gives you a good overview of the differences between Google Apps and Outlook.
When you start Outlook and connect to the Internet, Google Apps Sync begins downloading all the new messages, calendar events, and contacts that have come in since you last connected. The download is chronological, so if you last connected on Friday, and now it's Monday, Friday's messages arrive first. Your email, contacts, and calendar events all synchronize concurrently as separate threads (though email generally takes the longest to complete since there's more of it). You can tell that synchronization is happening when the Google Apps Sync icon in your system tray is spinning.
If you don't stay connected long enough to complete an entire synchronization, Google Apps Sync downloads what it can. When your connection resumes, it picks up from where it left off until it completes the synchronization.
ITS has prepared a Google Migration Checklist for Outlook users to help you migrate smoothly to using Outlook with Google Apps. When you're ready to make the move, follow these instructions to install Google Apps Sync for Microsoft Outlook.