Clutch or brake will disengage the cruise control.
You can accelerate and when you take your foot off the car will go back to its pre-set cruise speed (handy for overtaking etc).
Remember cruise control is made for highways and will hold a more accurate speed at 80kmh+. It is constantly checking your speed and refining it to hold it at your set point. If you go down a hill, your car speed can increase and when it can control the speed again, it can go under the preset speed.
If you are interested in how it works, it is essentially a PID controller
PID controller - Wikipedia, the free encyclopedia