Character in action is exactly what it sounds like. There's a character, and he or she (or it?) is doing something. It doesn't have to be something flashy, though it might be. You could, for example, start your story with a girl witnessing her father's hanging. That's flashy, and appropriate if the story revolves around what the girl witnessed and how it affected her. But you might also start with the girl answering the door to a man who tells her her father has been hung. Less flashy, but just as appropriate if your story has little to do with the hanging and much more to do with the girl. Both are character in action scenarios, but they are very different from each other.
Description of setting is a more subtle way to begin a novel, but it is no less effective. You might begin your novel by exploring the environment that is about to become center stage in your story, but if you do this, the setting must be important. It must mean something either to the story or to the character you're about to meet. If your story begins with a wedding (or a funeral), you might begin my describing the scene before getting to the people. Or someone might be riding in a car, and you describe the setting, typically as it relates to the character and his or her emotions. This can be as intriguing as a character in action opening.
Ultimately, you have to decide which type of opening suits your story best. Writing a novel isn't an exact science, so what works for one story will not necessarily work for another. Don't let anyone tell you that you have to begin with action or that you have to jump into the meat of the story with the first sentence. You must find what serves your tale best.
