Office Communicator is a business version of MSN Messenger with support for a wide range of communication types such as instant messaging, VoIP, PC-based voice, rich presence and video and enterprise telephony.
One of the big advantages of Microsoft Live Communications Server is the ability to connect to various IM networks such as AOL, Yahoo! and of course MSN.
"Communicator and Live Communications Server demonstrate how we are bringing integrated communications to life for our customers as rich presence and multimodal communications become an integrated part of the Microsoft Office System. Users now can view rich presence information for colleagues, partners and customers; select the most efficient mode of communication, whether it's IM, a phone call, a Web conference or e-mail; and instantly share ideas and information."Office Communicator together with Live Communications Server also feature encryption to make instant messaging safer.
Source: WebProNews