Box.net (often just called "Box") is primarily for synchronizing documents and files across multiple computers and mobile devices, similar to Dropbox. Free storage is limited to 50GB (1GB limit per file), or 500GB (2GB limit per file) for $15 per user per month. Keep in mind that the free service is intended for personal, noncommercial use only; it doesn't let you sync desktop files easily, and it lacks password-protected sharing, document version history, and other features offered by the paid service.
You can always upload files and manage them via the Box.net website. You can select files via a traditional dialog box, or drag-and-drop folders or files into the web browser. Paid subscribers can also install the Box Sync application to integrate access into Windows, similar to what Dropbox offers for free.
Box.net offers some sharing and collaboration features; for example, you can share via a web link or invite people to collaborate. You and others can leave comments on files, tag them, and make edits. Box.net also offers many add-on features and functionality via applications, which can help when integrating Box.net access into other sites or applications.