The Common Internet File System (CIFS) also known as Server Message Block (SMB) is a network protocol whose most common use is sharing files on a LAN. The protocol allows a client to access servers, files and printers on the LAN as though they were on the local machine.
Microsoft uses CIFS in all flavours of Windows to provide its networking capabilities (i.e.: Shares, Network Neighbourhood). Unix/Linux also use CIFS via Samba, and Apple has several clients and servers available. Thus, it's a fairly interoperable protocol.