Once you've finished processing your image, you can click done to save your edits. Edits are non-destructive, and you can go back in at any time and continue editing. You can also see your original, unedited image at any time by tapping on the image itself, and you can go back to the unedited image by tapping on the revert control that appears when you go to re-edit an image that you've made adjustments to.
Overall, the new editing functions are a welcome addition, and Apple appears to have integrated most of what was in the original iPhoto for iOS app. To be honest, I don't edit my photos that much on my iPhone anyway, and when I do I'm usually using my favourite app, VSCO Cam, but there have been times when the new controls have come in handy, and I've edited in the photos app rather than going to a third party option.
Behind the scenes
There are a couple of more important additions to the Photos app, and the underlying architecture that won't really become apparent until third party apps are shipping with iOS 8 support. The new APIs in iOS 8 allow for a few important changes to the way third party apps integrate with your camera roll.
The first is extensions, which was discussed during the WWDC keynote. Extensions will give you two new options in iOS 8. The first will allow you to send an image directly to a third party app, and the second will allow a third party app to insert controls and functionality into the photos app. During the beta period there were no apps available with this functionality, so I could test this out. I'll revisit this once there are third party apps out there supporting extensions
Another behind the scenes change, which got less fanfare, but is still a pretty big deal is the fact that third party apps can now access the camera roll directly, with your permission. In the past apps would have to import images from the camera roll and then export them back. In iOS 8 they can now write directly to the camera roll, so hopefully that means we wont end up with lots of little separate libraries in each of the different apps that we use.
The other headline feature of iOS 8 when it comes to photos is the iCloud features. With iOS 8 Apple gives you the option to store all your photos in the cloud, and access them across all your devices. Currently, as I'm writing this, this feature is in beta, and I only have iOS 8 on one device, so I haven't been able to test it. There doesn't seem to be any way to access the iCloud photo library via the mac yet, even in Yosemite. This may change closer to the release of Yosemite, but for now I haven't been able to test this properly.
Apple does give you the option to still use photo stream in conjunction with the new iCloud option, allowing you to still sync to Aperture and iPhoto for the time being. To enable this you need to go to Settings -> iCloud -> Photos and turn on "My Photo Stream"