If you've ever found yourself viewing a webpage in Safari on your iPhone and wanting to instead view it on the larger screen of your iPad or Mac--or, conversely, reading an article on your Mac and wanting to transfer it to your iPhone as you walk out the door--iCloud Tabs is for you. This feature syncs, between all your Macs and iOS devices configured with the same iCloud account, any tabs open on any of those devices. Instead of the hassle of emailing one or more URLs to yourself, iCloud Tabs lets you quickly view--on the device you're currently using--any tab open on any of your iCloud-synced devices.
To access this feature, you first need to ensure that all your devices are configured with the same iCloud account. On iOS devices running iOS 6, you configure your iCloud account in the iCloud screen of the Settings app; on Macs running Mountain Lion, you use the iCloud pane of System Preferences. On both platforms, be sure Safari is enabled in the list of data to sync. (Note that if you've got multiple iCloud accounts configured on your iOS device or Mac, only the main account--the one configured in iCloud settings, rather than in the Mail, Contacts, Calendars screen in iOS or in the Mail, Contacts & Calendars pane in OS X--can use iCloud Tabs.)
Once configured, iCloud Tabs automatically syncs open browser tabs between all your devices--assuming those devices have Internet access, of course. On the iPad, there's an iCloud icon in the Safari toolbar; click on that to get a list of the tabs open on your other devices. On your iPhone, you tap the Bookmarks button and then select iCloud Tabs. (If you're already in a sub-list in the Bookmarks list, tap the left-facing arrow until you get to the top level, titled Bookmarks.) Tabs are grouped by device, and each tab's entry shows the name of the webpage and its URL; tap any item in the list to open that webpage on your device.
On your Mac, you access synced tabs by clicking the iCloud Tabs button--which looks like a cloud--in Safari's toolbar. Tabs are similarly grouped by device, although on the Mac they don't display each webpage's URL.