It's almost impossible to break our own neck by cracking it. But you can still do some serious damage if this is a regular habit, so I'd try to cut it out if I was you.
From what I've read, it seems like the reason you hear cracking or popping noises is because the movement creates pockets of air where fluid from your joints fills by turning into a gas. That's what makes the 'pop' sound.
What it means for your neck is that you could be causing damage to the line of the artery around the vertebrae, and that could cause stroke. And that could kill you!
You could also do nerve damage if you crack your neck over and over again for a long time. Doesn't sound worth it for a moments relief that could achieve by getting a neck massage anyway.