On your Mac go to Photos > Preferences > iCloud and check the box labeled "iCloud Photo Library" then if you have plenty of storage available click the radio button labeled "Download Originals to this Mac", if storage space is limited click "Optimize Mac Storage".
On your iOS device(s) go to Settings > Photos and turn on iCloud Photo Library. Below that if you have enough storage on your iOS device for all your fill size images select Download and Keep Originals, if not select Optimize iPhone Storage. Turn on Upload to My Photo Stream. If you have a generous data plan turn Cellular Data ON, otherwise your libraries will only synch when you have a WiFi connection on your iOS device(s).
With these settings the contents of your Photos Libraries on all your devices and iCloud WILL for all intents and purposes BE IDENTICAL and will automatically remain that way. Any changes or edits on one device will within minutes be reflected on iCloud and every device attached to your iCloud account. You can select a group of images on your Mac or on your iOS device and do what you will with them and that action will be reflected on iCloud and all your other devices. The only reason to use Photos on iCloud would be in a case where you wanted to show your Photos and did not have your computer or iOS device with you.
