What is the "cracking" or "popping noise"?

Your bones are not "cracking". The actual noise is called a cavitation. When a joint is adjusted, a gas bubble may escape causing a popping noise, the same noise you hear when you crack your knuckles. It's very similar to the release of gas bubbles when you un-cork a champagne bottle or open a can of pop. Here is a fun video explaining that happens when you "crack" your knuckles: 

Does the "popping" happen with all adjustments? Is is still a "good" adjustment if you don't hear the "pop"?
Although it may be satisfying to hear the "popping" noise with an adjustment, you may or may not hear a cavitation ("popping"). Just a week ago, I had a new patient who came in with acute severe neck pain. She could not turn her head and was miserable with pain. Due to the acute nature of her condition, we could not do a traditional adjustment involving my hands and turning of the patient's neck. I used the Activator (a tool used to gently adjust without the patient having to turn her neck) instead. Typically, with Activator adjustments, you do not hear a caviation although the patient really wanted to hear it. She had been to chiropractors before and associated successful adjustments with the "pop". We also adjusted her lower back with the Activator. At the follow-up visit, her neck pain had improved and she re-gained a significant amount of her range of motion. She was in a much better mood as you can imagine. And her lower back? The improvement was "night and day". She had been suffering with chronic low back pain, causing her pain in her daily activities such as putting on her socks, but that pain was gone.