GPU Tweak is ASUS' implementation for GPU overclocking and monitoring. ASUS' version is one of the only overclocking utilities which does not run on a RivaTuner code base.
ASUS has packaged GPU Tweak with a red and black skin, utilizing GPU-z for GPU information, and a large number of monitoring statistics. ASUS offers an "easy mode," which has a reduced overclocking range, which should yield a "safe" range for beginners or casual overclockers to give their cards a boost without worrying about frying their card. They've also given you options to link or un-link voltage with GPU clock settings. The linked setting uses a "conservative" voltage jump to ensure stability, which is generally higher than necessary. This setting is nice in that it gives you a good starting point for a given clock, then you can work your voltage back from there to improve efficiency or temperatures. The "Advanced Mode" also gives the option to independently control voltage and GPU clock for more seasoned or comfortable overclockers.
ASUS also offers an "Overclocking Range Adjustment" if you're looking to push the voltage and/or clocks higher than the safe mode allows. ASUS has been a leader in fan controls on their motherboards, and they also offer user-defined fan settings based upon core temperature, giving you added flexiblity to control temperatures (and the impact on overclocking), as well as the noise profile.
The monitoring portion of the will prove very useful, as it offers a quick look at the entire state of your card as you use it. Temperature curves along with fan speed can be used to fine-tune your thermal and acoustic performance. Ensuring your card is being fully utilized and that the clocks are hitting their set targets is also nice to have. The monitoring software may also be used to log all of the statistics so you can track GPU states through GPU or stress testing.