Windows Server 2008 Server Roles and Server Features
In Windows Server 2008, a server role describes the primary function of the server. Administrators may choose to dedicate an entire server to one role or to install multiple server roles on a single server. For example, the DHCP adn DNS roles could be installed together on one server
A Server features provide supporting functions to servers. You can add features to improve the functionality of installed roles and not as the primary function of the server. For example, failover clustering is a feature thta is installed after specific roles, such as file server. This is done to make the File Server role redundant.
There are ~20 server roles in Windows 2008:
Active Directory Domain Services
Active Directory Federation Services
DHCP Server
Web Server (IIS)
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There are ~30 features avaliable in Windows Server 2008:
Background Intelligent Transfer Services(BITS)
Windows BitLocker Drive Encryption
Microsoft Message Quering (MSMQ) Services
Storage Manager for Storage Area Networks (SANs)
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You can use Server Manager to configure server roles/features.