Transmission Remote GUI is a remote graphical user interface. It basically is an application that lets you control Transmission remotely. Transmission is a BitTorrent client application for the Mac. The remote GUI is available for Windows, Linux and Mac OS X. This application is supposed to come with more features than the stock remote interface that comes with Transmission.
In my testing, I installed Transmission on my Mac and then proceeded to activate the remote features. After that, I installed Transmission Remote GUI and I added 127.0.0.1 as the machine's IP, because I was testing on my own computer. If you are trying to control another computer over the Internet, you might have to do some port-forwarding.
Transmission Remote GUI found my Transmission app instantly and it didn't have any problems adding and controlling torrents. Graphically, this GUI looks like older versions of uTorrent. The General tab shows information like status, downloaded and uploaded data, speed limits, seeds, tracker, peers, etc.
Some of the things you can do via this GUI are: add, delete and control torrents, set speed limits, tweak Transmission settings and look for downloaded torrents.
In short, this is a great app for controlling Transmission remotely. I wouldn't really know whether this is better than the included GUI, because I haven't personally tested it.