IMO iCloud is initially confusing, at least it was to me. So iCloud is generic speak for Apple cloud. iCloud in general is a device syncing services that does have a lossless copy in the cloud, so all your relevant data is mirrored between your devices. Very important, its mirrored. So if you delete it anywhere, iCloud.com or your iPhone it disappears off the one you didn't touch.
To me that sounded odd and almost pointless at first, however in the settings there are various optimize storage options. So for example when I take a picture on my iPhone its saved to the Photos app which uploads to the cloud and to my iPad and Mac. With optimize storage on my iPhone it will upload to the cloud and other devices and just keep a thumbnail on my iPhone. When I tap on it it will download the original copy. This makes my 50gb photo collection use about 1-2gb on my iPhone. And I have access to every photo I've taken over the last 10 years handy...anyway...
iCloud Drive and the associated iOS Files app on your iPhone is just a place to drop files. Its like Windows Explorer or Finder depending on which you are familiar with. If you download something it will be in there under Downloads too. Its very basic file storage that is accessible from all your devices, again if you delete it, its gone everywhere.
iCloud Photos and the associated iOS Photos app on your iPhone is where all the Photos you take with the camera app go. iCloud Photos once turned on (and every Apple device with a photos app) will get a copy of the photo within a minute or two of taking it. In leu of sounding like a broken record, if you delete the a photo out of your Photos app it will delete it everywhere.
Messages in iCloud sync all your messages between all your Apple devices. The benefit to this is if you have a lot of devices it removes the message portion from the backups and into a single location. Plus like photos and such you can optimize device storage and the original is saved in the cloud.
iCloud Mail is a iCloud email address I believe. Calendars, Notes, Contacts, Pages, Numbers etc all sync...
Hmm yep, still sounds confusing...