iCloud is the name used by Apple for the service that synchronises data between your Apple devices.

In System Preferences, click on the Apple ID section (older OSs click the iCloud section) and then iCloud in the side bar. Tick which apps you want to sync e.g. Contacts, Calendars, Reminders etc. Repeat this is Settings on your iOS devices. When you make a change e.g. new telephone for a contact or new calendar entry, Apple automatically send it to all your other devices.

iCloud Drive

iCloud Drive is the off site storage that's part of your free space on Apple's servers. Currently this is 5GB but you can buy more if you need it.

Tick this option in System Preferences (see above) and then in Finder Preferences, choose to show iCloud Drive on the side bar. Now when you open a Finder window and click the iCloud Drive option, it will display all the files you have stored there. Use it like any other folder on your Mac to keep files safely online. Turn it on in Settings on your iOS devices and they will have access to all these files.

Note that with iCloud enabled apps, Apple sync your data automatically but with iCloud drive it's "manual" - i.e. you manually upload and download to and from your devices.