Curse Client is an application that keeps tracks of your addons and updates them as necessary whenever the author of that addons uploads an update. In a nutshell this is your "addon updater."
You can download Curse Client from Curse.com which you will need to register an account to sync your addons!
Or you can download the installation >Curse Client and register that way.
Either way, I highly reccommend that you get and use Curse Client if you have numerous addons!