On the common Windows versions, Linux or Mac OS X.

No, a client can also back up other computers in the network (e.g. via Windows share/ssh/nfs).

Yes, you need a Windows client for all Windows computers and a Linux client for all Unix-based computers. Backing up with one computer from both systems using the smb protocol (Samba) is not recommended.

Yes, the client also runs there.

No, but we recommend running a client on every computer that is to be backed up. In some cases, we recommend using a separate computer for larger networks.