Version 1.7.6 of Minecraft was announced yesterday. The new version fixes some bugs, and makes some changes that will eventually allow players to change their player name.
The version 1.7.6 client is not compatible with earlier versions of the server. This is mostly due to changes in the player profile format. Player profiles should be updated to the new format automatically by version 1.7.6 servers.
The JDRGaming Minecraft server will be updated to the new version later today.
See the Minecraft page on this web site for more details.