Terminal Server (Remote Desktop Services) licensing.
When Remote Desktop Services role is installed you will be given 3 options.
Configure Later - You can postpone your decision and simply select Configure Later. You'll have a grace period of 120 days to configure licensing and select a licensing mode. It's common to choose this option early in the deployment cycle and then configure the RDS CALs once you've tested things out. Although you have a grace period when RDS will function normally, after the grace period ends, RDS will no longer accept connections if licensing is not configured. The grace period lasts for 120 days or until the first permanent RDS CAL is issued by a license server, whichever occurs first.
Per Device: A per-device CAL is issued to a client computer or device. If the licensing mode is set to Per Device and a licensing server has been configured, the licensing server will issue the device a temporary license the first time the device connects. The second time the device connects, the licensing server will attempt to issue it a permanent license. The licensing server will enforce per-device CALs. If a per-device CAL doesn't exist for the device and an RDS CAL isn't available to be issued, the connection will be blocked. MS says to use per-device CALs if multiple users will use the same device to connect to an RD Session Host server. This works well for permanent thin client stations.
Per User - A per-user CAL allows a user to connect to an RD Session Host server from any number of devices. The license server doesn't really track the per-user CALs. This can make things both easier and more difficult. It's easier to manage on a day-to-day basis because the RD Session host server won't stop users from connecting. However, administrators will want to make sure they are compliant and the appropriate number of CALs have been purchased.
It is possible to configure the maximum connections supported by the server to coincide with the number of purchased CALs. This is done on the Network Adapter page of the RDP-Tcp Properties in the Remote Desktop Session Hosts Configuration console.
Microsoft says you should use a per-user CAL if users will connect to an RD Session Host server from multiple devices.
If an unlicensed client connects to a Terminal Server for the first time, the Terminal Server issues the client a temporary Terminal Server Client Access License (CAL) token. After the user has logged into the session, the Terminal Server instructs the License Server to mark the issued temporary Terminal Server CAL token as being validated. The next time the client connects, an attempt is made to upgrade the validated temporary Terminal Server CAL token to a full Terminal Server CAL token. If no license tokens are available, the temporary Terminal Server CAL token will continue to function for 90 days. The license is stored in the client's registry.
Thirty-two bit RDP clients store their license under the key HKEY_LOCAL_MACHINE\\Software\\Microsoft\\MSLicensing.
Some URLS where this information came from :
Removing Terminal Service Licenses from an RDP Client: http://support.microsoft.com/kb/187614
Troubleshooting Remote Desktop Licensing Issues: http://technet.microsoft.com/en-us/library/hh553162(WS.10).aspx