Everything backs up on iCloud, the PIM side what they used to call back in the days Personal Information Managers, which include contacts, calendar, tasks, notes, etc. iCloud as other services out there (e.g. Google), offer more options on what to back up. Depending also on what type of service you use for your PIM.
I personally use google so all my contacts, calendar, chrome bookmarks, google drive data is stored and managed by google, all I do is activate on my google account contacts and calendar and all my contacts and calendars sync.
Ask a more detailed questions, what type of service do you use? Are you planning to use iCloud services for everything? Or do you currently use another service out there for managing your life?