Your own files
Opened from the start page, the apps work on files from your computer: Save and Export download them again, and nothing is uploaded. Only files you share are stored on this server.
Office formats
The document, spreadsheet and presentation editors support .docx, .xlsx and .pptx. These are early (alpha) tools: keep backups and check important files in a mature editor before distributing them.
Shared files (beta)
Create edit and view links for the same file. Others can see who is online and receive saved versions. This is version-based collaboration, not yet simultaneous live typing; conflicting saves are retained separately. Account-based permissions are planned.