There are many new features available to you in iOS 8 in the Photos and Camera apps. When you're viewing an individual photo, one nice new feature is the ability to designate it as a favorite. Then those photos that you've identified as favorites are automatically collected into an album titled Favorites.
To mark a photo as a favorite, simply tap on the heart icon, shown here at the top in the Photos app on my iPad. (I believe it's at the bottom of the photo on an iPhone.)
Those that you've indicated as favorites show up as a Favorites album in the album view in Photos.
Open that album, and you'll see all your favorite photos.