One way of classification is yin vs. yang. There is a yin way to practice chi kung and there is a yang way to practice chi king. When a learned practitioner says he practices qigong (= chi kung), chances are that he is mostly practicing the yin aspect, whereas when another says he practices neigong, chances are that he is mostly practicing yang aspect. And assuming that he is a seasoned practitioner, he will practice both the yin and yang side of chi-kung. My dear reader, if I have not yet frustrated your intellect, please read on....
The mother of chi-kung is embracing a tree zhan zhuang. As I explained in other posts, in zhan zhuang, the most important requirement is to establish a yang position at a practitioner's extremity, meaning a pair of stretched hands. To balance this yang position, the body itself should be relaxed and heavy （沉). Chi will flow. A good foundation in zhan zhuang can greatly help a practitioner to move further.
The next logical forward step in one's practice is do the yang aspect of chi-kung which for lack of a good term, I shall call it neigong (or neigung). The principle is like this: With a pair of well connected stretched hands with a relaxed and heavy body, one shall use connected force to forcefully close one's fists, or close them into claws (like Tiger claws of tai-chi 24 styles neigung - the first movement form of the yang series). This action will forcefully strengthen one's muscles and tendons. Incidentally similar neigung principle is also used in advanced katas of some external martial arts. Examples are Tei Xian Chuan of Hung Gar 洪家鉄綫拳 (iron wire fist), or SanChin 三戰 of karate.
The Yin forward step is more difficult to comprehend for practitioners without previous exposure to Taoist or Buddhist meditation. The gist of it is to (after establishing a good chi-connection using zhan zhuang) gradually and slowly diminish the stretched-state of one's hands. In tai-chi lingo, it is a minus-way 減法 of doing tai chi. Unfortunately, even nowadays, many tai-chi practitioners still didn't realize there exists this minus-way of doing tai-chi form. In the past, it was only reserved for a group of inner circle students. The hands will be changed to a yin point whereas the chakras or other meditative points of choice will be changed into yang point. This might sound confusing to a martial artist or muscles-practitioner, but for a Taoist or Buddhist meditation, this way is THE way of doing advanced meditation. For mind-body health benefits, the yin forward step can facilitate chi to penetrate to the deepest organs as well as to totally shelter our body through chi-filling our skin. On the latter chi-filled skin practice method, psychologically a practitioner can practice ego protection, and physically he can practice taki-shu-gyou 滝修行 or Tummo yoga. The sky is the limit.