iCloud is more deeply integrated into Mountain Lion in a way that has profound implications for the way you work. Apps such as TextEdit and Preview are able to save documents directly to iCloud. Those documents are then pushed to your other devices, as long as they're online, so you can pick up your work anywhere.
Online documents are organised in the iCloud Document Library, a new dialog that appears when an app first opens, and when you choose File > Open. Options at the top-left switch between browsing online documents and your Mac's local storage. Documents in iCloud can be grouped in folders, which use the same visual styling as Launchpad and the iOS home screen. Folders can't be nested.
Files already stored on your Mac are easily moved to the cloud. Drag them from the Finder into a document library, or open them, choose File > Move To… and pick iCloud as the destination.
Awkwardly, there's no universal document library to view files irrespective of the app that created them. When you need to share, say, a PDF and a TextEdit document, you must begin sharing the former from Preview's library, then open TextEdit's library and drag the desired file onto the other app's Share sheet. Documents are grouped in an almost entirely artificial way – by application.
It's a cumbersome way to work, and it doesn't fit with human ways of organising work by project. In moving away from complex folder hierarchies, you soon realise that Apple's simplification is a step too far.