How does iCloud work. Newbie question.

Hi all,

so I was using MacPro for a while, but just realized I had iCloud. So far I was using apps like OneDrive, Dropbox, Box. So what is the deal with the iCloud - obviously I can upload only online, not like Dropbox for instance, when you put something in the folder of the app and it uploads it automatically. Also I noticed that I can't upload folders. If you can give me a guide or something for people who are not so into the app would be great. Thx