The issue of "neck cracking" as a form of treatment used by chiropractors can be a bit controversial. Many mainstream publications have highlighted the fact that there have been some adverse effects from the procedure. These range from the terrible like a stroke, to more subtle changes that are barely noticeable but still negative.
However, these really bad side effects are very rare so you need to realize that you stand a better chance of being hit by a bus when crossing the street, than you do of any serious consequences from a neck treatment. Here is an article that discusses the pros and cons of "neck cracking". http://lifechiropracticcentre.ca/truth-about-neck-cracking
Here is a video that shows the treatment being performed