What this does it keeps your Mac Address Book Contacts synced with your Google/Yahoo Mail Contacts automatically. So that when your working away from home or on vacation you can log into google/yahoo mail and have all your contacts email
addresses there, and up to date. It does not sync email.

Apple has something called Mobile Me which you can purchase for PC and Mac. This is what Mobile Me does:

Mail, Contacts, and Calendar. All your stuff. Always in sync.

Maybe you have a Mac at home, a PC at work, and an iPhone. MobileMe uses push technology to keep them all up to date. Make a change on one, and the rest are automatically updated.

Gallery.
A better way to share photos.

With MobileMe, you can create a beautiful web gallery of your photos in just a few clicks. Add shots from your iPhone and flick through them with the new MobileMe Gallery app for iPhone and iPod Touch.

iDisk.
Your hard drive on the web.

iDisk makes it easy to store and share files online so you can access them from any computer. And with the free iDisk app for iPhone and iPod Touch, you can view and share files even when you’re on the go.

Find My iPhone.
Locate your missing iPhone.

Don’t know where your iPhone is? Let MobileMe help you find it with Find My iPhone. Display its location on a map, remotely set a passcode lock, or protect your privacy with Remote Wipe.

When you purchase Mobile Me they also give you a mobile me.com address where you can set up email addresses and have them come to your home computer, your iphone, or accessible from anywhere you are.

If someone doesn't want to purchase Mobile Me then syncing of the mac address book and google/yahoo is a good alternative.

I have Mobile Me and I love it.

There is a free 60 day Trial for Mobile Me for your PC or Mac which you can find here Mobile Me Free Trial