iCloud is similar to Dropbox, except it mainly targets data between Apple devices. Unlike Dropbox, you cannot openly host any files and access them anywhere, it's strictly Apple applications such as Pages, Numbers, etc, and it's also part of Apple's security protocol for finding lost/missing/stolen devices that are tied to your account. It also allows you to backup user-end data to an extent, such as iMessages, Notes, etc. You can browse through the features individually here in greater detail