Revisions lets you see all file edits in your Dropbox, and restore old versions of files and entire folders with the click of a button.
With Revisions you can also restore the contents of a shared folder after your colleague deleted a large bunch of files to make room in her own Dropbox, inadvertently deleting your copies of the files in the process and keep track of what files you or your team worked on in any given week, and exactly what you accomplished.
In essence, Revisions provides for "track changes" functionality for all the files in your Dropbox.