Windows Server 2008 is entirely roles-based in how you manage it. There is no longer a concept of installing many separate and unrelated components the way there was in older server operating systems. In Windows Server 2008 you decide what role you wish your server to play and the Server Manager tool provisions it with the necessary software to do that.
Microsoft distinguishes between roles and features. A role is a collection of software that collectively enables the server to provide some service to the network. Generally, a role is what you bought the server for. An example of a role is “Domain Controller” or “Application Server” (such as a Web server with application frameworks). Often a role can be installed in one step, but may require significant configuration to function the way you want it to. For instance, you can easily promote a server to become a Domain Controller but unless you add user and computer accounts to it, having a DC does not do much good.
Features, on the other hand, are simpler. They are things that many servers need, and that there is a good reason to have, but that you may not consider as the primary reason you bought the server. They do not describe the server in the way roles do. In some cases, features involve no services at all, as is the case with the Recovery Disk feature, which is just a tool. Other features do have services associated with them. In many cases, a feature is simply a way to surface the installation of a service to an administrator. Many of the things you saw under the Programs And Features Control Panel in Windows Vista are listed as features in Windows Server 2008. In some cases, the distinction between roles and features is quite unclear. For example, Dynamic Host Configuration Protocol (DHCP) and Domain Name Service (DNS) are both roles, but Windows Internet Name Service (WINS) is a feature. WINS keeps track of DHCP-assigned Internet Protocol (IP) addresses and provides the same services for legacy Windows networks that DNS provides for up-level ones. As you go through the features, you will find that some optional components slated for deprecation are listed as features. This is now the only way to install them.
Some roles require certain features to work. For instance, the Active Directory Rights Management Services (AD RMS) role requires the Windows Internal Database feature (Microsoft SQL Server 2005 Embedded Edition). Likewise, some features may require other features, as is the case with Windows System Resource Manager (WSRM), which also requires the Windows Internal Database.
In many cases, the links between features and roles are quite intricate. For example, consider the case of an Application Server. If you attempt to install that role on a freshly laid down server you are asked to install two features—.NET Framework 3.0 Features and Windows Process Activation.
Unless you specifically modify the default selections you will get all the roles and features installed that are needed for the essential services of the particular role, along with the basic feature support. You can uncheck things you do not want, but if you uncheck something that you must have, the system will inform you that you cannot install the role in that case.
Source of Information : Microsoft Press Windows Server 2008 Security Resource Kit