The Windows client is the most full-featured of the trio of Verizon clients. You can schedule backups, including initiating one immediately, examine files that are in your repository from a Windows Explorer shared drive, and change the number of versions saved (more on that later). Windows XP 64-bit clients aren't supported, and you'll need .Net Framework v2 or better, XP with SP2 and 2000 with SP3 to make it all work.
The Mac client runs on OS X 10.5 or better and can only schedule backups in the future by hourly or daily, (meaning you can't kick off an immediate backup manually like you can with Windows). It has a slightly different interface from the Windows version. This isn't much of a problem unless you have users who run both kinds of desktops.
The Web client can set up a shared space, similar to Box.net or Google Docs, where others can gain access to your files. You create sharable folders in its Online Files section. These can be seen in Windows Explorer as a separate sharable folder just as any other network drive. You can drag and drop files from the Web client, or add them by copying the files inside Windows Explorer.
You can also send a collaborator a special Web link via an email message that will enable them to download the file to their desktop. You just click on the options icon next to the file, select share, and enter their email address. You can make the file read-only, or have the link expire after a particular time period.