The advantage of iCloud Photo Library on an iOS device is that your photos size will be optimized for your device if you are low on storage space on that device. When you open an optimized photo it automatically downloads a full resolution version before doing anything with it, it's all automatic. Deleting or editing any photo will automatically sync to iCloud and any device you have iCloud Photo Library turned on.
Photos Library on your Mac
iCloud Photo Library stores photos and video from only one library on your Mac, (Photos Library.photoslibrary) You can have multiple photo libraries, but you can have only one System Photo Library working with iCloud at a time.
My Photo Stream with and without iCloud Photo Library
With My Photo Stream turned on, new photos taken with your iOS device are uploaded to iCloud when your device is connected to the Internet via Wi-Fi. The new photos appear automatically in My Photo Stream on your all your devices when iCloud Photo Library is not enabled. When iCloud Photo Library is turned on, they become part of your photo library and appear in All Photos. (iOS 9 or iOS 8)
Photo Stream: Photo Stream stores the last 30 days of photos automatically, up to 1000 photos, and pushes them to all your Apple devices. The minute you take a photo with your iPhone or iPad it will be stored in iCloud and it will put them on your computer the next time your computer is online.
Photo Sharing: With iCloud Photo Sharing, you can share photos and videos with just the people you choose, and you can let them add their own photos, videos, and comments. Friends and family can subscribe to your shared albums to view them on all of their devices, leave comments, and get notified automatically when you add new photos and videos. Photo Sharing does not count aginst your free 5 GB storage.
When you delete a photo from My Photo Stream on an iOS device, Mac, or PC, that photo will be deleted from the My Photo Stream view on all your devices after you connect each device to Wi-Fi.
The following photos won't be deleted when you delete photos from My Photo Stream:
Photos that you copied to the Photos tab or Camera Roll on your iOS device
Photos that you imported into your iPhoto or Aperture library
Photos that you copied into a folder on your PC that aren't part of My Photo Stream
Turning Off iCloud Photos Libraries: If you have Optimize Storage turned on, you might not have all of your original photos and videos on your device. To download the original photos and videos on your iOS device, go to Settings > iCloud > Photos and select Download and Keep Originals. On your Mac, open Photos, choose Photos > Preferences, then select Download Originals to this Mac. After the photos have downloaded completely, which might take a long time, then you can turn iCloud Photo Library off. You can also select the photos and videos that you want to download from iCloud.com.
To use Mail syncing with iCloud you have to have an Apple mail account which will have the “name”@icloud.com- address. This account will not sync with your existing mail accounts. Once it is set up it will sync with all your Apple devices. It will keep all your contacts and calendars synced.
iCloud Drive lets the app on your device access any stored document that can use iCloud Drive, from any device you have iCloud drive turned on. It syncs documents for all devices. You can store any type of file in iCloud Drive, as long as it's less than 15 GB in size and you don't exceed your iCloud storage limit. There's no restriction on file type.
Pages, Numbers, and Keynote upload files to iCloud regularly to make sure you don’t lose any changes. Other apps upload your files to iCloud only after you save them to the iCloud Drive folder. The app folders in iCloud Drive are created automatically for apps that support iCloud Drive on any of your devices. You can create new folders to use.
Removing a document from iCloud deletes it from iCloud Drive and from your other devices set up for iCloud Drive.
Before turning off iCloud Drive, you can copy documents from iCloud Drive to your Mac.
All your iTunes purchases will be stored with iCloud. If you have a lot of music in your iTunes library that you copied from CD’s, then to get iTunes to store them you can use iTunes Match, for $25 a year.
iCloud Keychain keeps your Safari website usernames and passwords, credit card information, and Wi-Fi network information up to date and Mail, Contacts, Calendar, and Messages passwords up to date across all of your Mac computers.
iCloud uses 128-bit encryption--the same level of security employed by major financial institutions--and never provides encryption keys to third parties.
Accessing Your iCloud Data From Windows
There are two ways to access synced iCloud data on Windows: via the iCloud Control Panel desktop application and via the iCloud website.
Download the iCloud Control Panel for Windows, or go to icloud.com.
You can’t view your Photo Stream on the iCloud website, you would need to install the iCloud Control Panel app for windows.
5 GB Free: You get 5GB of free iCloud space, Your iCloud storage is used for iCloud Backup, iCloud Drive, iCloud Photo Library, iCloud Mail (your @icloud.com email account), and data from apps that use iCloud. Your music, applications, iBooks, TV shows and Photo Stream don’t count towards the 5GB. You get those free. And you can get more space for a fee.
Family Sharing: To share between people with different Apple ID accounts
Family Sharing makes it easy for up to six people in your family to share each other’s iTunes, iBooks, and App Store purchases without sharing accounts. After you set up Family Sharing, all the eligible songs, albums, movies, TV shows, books, and apps ever purchased by family members are immediately available to everyone else in the family. The content appears automatically in the Purchased tab in iTunes, iBooks, or the App Store for each family member. Just select the family member whose collection you’d like to browse, then download and play the content you choose. Other family members can access your collection in the same way. If you want to keep some purchases private, you can choose to hide individual items.