Taken directly from microsoft's website, this should help you out quite a bit in understanding them.
Link:
http://technet.microsoft.com/en-us/library/cc779407%28WS.10%29.aspxBest practices
Consider the following factors when configuring and tracking licensing for your organization:
* Per Device or Per User licensing mode is the best option if your clients frequently use multiple servers on the network.
The Per Device or Per User licensing mode enables all network devices or users to access all the servers on a network. The number of simultaneous connections to any server is unlimited. Per Device or Per User is the normal licensing mode for a server product that is installed on multiple servers in a network.
For more information, see the Microsoft Web Site .
* Per Server is the best licensing option when a server product is installed on only one server accessed at any time by no more than a subset of your users.
Per Server connections to a particular server are allocated on a first-come, first-served basis, and are limited to the number of Client Access Licenses allocated to the server. This might be more economical for a single-server network or for a server product that is accessed on one server by only one department or group in an organization. As your network grows, you are allowed a one-time conversion to Per Device or Per User licensing.
For more information, see the Microsoft Web Site .
* If you install a server product on a single server and you are uncertain which licensing mode is appropriate, choose Per Server.
As your network grows, or when you install the product on additional servers, you can make a one-time switch from Per Server to Per Device or Per User licensing. The licensing agreement does not permit the reverse switch from Per Device or Per User to Per Server licensing.
* When the number of users is different from the number of devices, create a license group to allocate licenses for Per Device or Per User licensing.
Per Device or Per User licensing requires one Client Access License for each device. However, the License Logging service assigns and tracks licenses by user name. You should create license groups to correctly allocate licenses and track usage when multiple users share one or more devices or when one user has multiple devices.
For more information, see License groups.
* To replicate license data from a server running Windows NT 4.0 to the licensing database for a Windows Server 2003 operating system site, specify the site license server as the enterprise server for replication on the Windows NT 4.0 domain controller.
By default, Windows NT 4.0 servers replicate to the domain controller or to an enterprise licensing server. On all servers running Windows Server 2003 , the License Logging service replicates automatically to the site license server. In the replication options for a Windows NT 4.0 domain controller, always choose to replicate to the enterprise server and always specify the site license server as the enterprise server.
For more information, see:
o Configuring licensing replication
o Configuring Windows NT 4.0 licensing replication
