Photos and videos captured by your iPhone, and photos saved from a received email, from a website, or from a received MMS are stored in the Camera Roll. The Camera Roll is for temporary storage as with any other digital camera and as with any other digital camera, photos/videos in the Camera Roll can and should be imported by your computer.
Photos transferred from your computer via the iTunes sync/transfer process - selected under the Photos tab for your iPhone sync preferences with iTunes, are stored in the iPhone's Photo Library. If you transfer multiple albums or folders of photos, all photos in all albums or folders are stored in the Photo Library. The photos in the albums or folders below are not duplicated. The photos in the albums or folders below include a pointer to the original photos stored in the Photo Library. This way you can view photos in an album or folder only be selecting the album or folder below Photo Library, or you can view all photos in all albums or folders transferred from your computer by selecting Photo Library.
You cannot transfer photos from your computer to the Camera Roll. All photos transferred from your computer are stored in the Photo Library. You can have separate albums or folders of photos transferred from your computer by creating the albums/folders of photos on your computer, but all photos transferred from your computer are stored in the Photo Library.
