What’s the difference between iCloud and iCloud Drive?

Apple's cloud service is called iCloud. When you create an Apple ID on an Apple device, you also get iCloud Storage of 5GB capacity for free. If you need/want more storage, you have to subscribe on a monthly-basis. You can use this storage to upload/sync/store/backup data from your iPhone (and/or other devices). This will depend on how you set it up.

(for eg. you can sync Contacts/Messages/Photos/Notes/Keychain, save certain app data and settings, and backup your iPhone, etc.)

All devices where you have signed-in with that Apple ID, can be setup to use the same iCloud Storage. So all these devices could share the same data syncs, etc. Also in this way iCloud mirrors what is on each device. (for eg. if you have taken a photo with your iPad, the same photo will also be uploaded to iCloud and iCloud will push it onto your iPhone, so it will be available on your iPhone too! ... depending on how you have setup iCloud on each of those devices.)

iCloud can be accessed on each of the devices that you have set it up ON. You can also access it by logging on to iCloud.com on any web browser on any computer.

iCloud Drive can be thought of as another feature/app of iCloud. It is a "special" folder which is created on your device. You can choose to store various files within this iCloud Drive folder. If and when you do so, because iCloud Drive is a part of iCloud, the files will be upload to iCloud ... and iCloud in turn will push it onto all devices connected to it ... so the files will become available on all those devices too ... as long as all those devices are connected to iCloud and have the feature iCloud Drive ON.

(Note: You do not have to turn iCloud Drive ON for all devices if you do not want the data on all devices ... it is just an option)