My knuckles, knees, ankles, and back often pop. I can feel when they need to be popped, so it's become a habit for me to pop them. Is this a bad thing? What's happening? Is my cartialige wearing away?

When you 'pop' them you're pulling bones apart and bubbles form in the fluid that's between them and they pop. That's the popping noise.
I crack my back, fingers, wrists, ankles and toes; nothing bad is happening. There's no health problems that come with popping your joints.