Apple first introduced the Photo Stream feature when it debuted the iCloud and now with iOS 6, it's introducing the ability to share Photo Streams. Photo stream synchronizes your photos on all your devices and now you can share those Photo Streams with friends. When your friends see new streams pop up, they can view and comment on them.
The new feature will show up as an additional tab in the Photos app. You simply select the the new cloud tab and choose whatever photos you want to share and who you'd want to share them with. You can also edit the share settings afterwards through the Photo app.
Your friends can comment on the photos directly within Photo Stream. The photos can also be viewed via iPhoto and Aperture, as well as on Apple TV and on Windows.