There is quite a few new Lync server roles built into Microsoft Lync, and not lots of information about what they are for, so here is an overview of each Lync 2010 Server Role, focused on their intended function in the Lync family (or “site” as the new terminology goes).
Archiving and Monitoring - Just like the name says. This server role monitors your Lync Server usage. Archiving archives IM conversations, Group Chat and conference logs.
Audio/Video Conferencing - Conferencing is integrated into Lync Server; a separate client (like Live Meeting was) isn't necessary. This server controls that integration.
Central Management - Main configuration server. The Central Management Store provides a master configuration database that sends configuration information out to all the servers deployed.
Director - The Lync Director server regulates user pools. It's usually on the front-end server.
Edge Server - Like they did before, Edge Servers make communications with external users possible. Lync Edge servers have also added DNS Load Balancing (helps reduce the need for Hardware Load Balancers).
Group Chat - The Group Chat feature allows users to discuss topics over time, with those discussions saved and searchable. Think of it like a bulletin board or discussion forum, built right into Lync.
Lync Web Application - Maintains the new Silverlight-based Lync Web App client. In case you conference with non-Lync users who'd like to join in without installing extra software.
Mediation - Handles mediation between servers and gateways. If there's a break in Lync's communication, the Mediation Server Role allows the call to bypass itself and flow from the Lync Server directly to a gateway or IP-PBX.
Reach Application Sharing - As the name implies, this role handles sharing of applications between users while chatting or conferencing.
Survivable Branch Appliance - This role helps keep remote offices connected. If there's a break in communication (say the network goes down), the Appliance Role will route calls through a local gateway out to the public phone network. So calls can continue even while the network's being fixed.
Unified Communications Application Server - As you might expect, it helps with recording voicemails & passing them to Exchange.
Web Conferencing - Provides a foundation for hosting Web conferences (with integrated audio/video from the Audio/Video Conferencing Server).