Yin is spiritual energy and Yang is physical energy. You need to manipulate both spiritual and physical energy to mold jutsus.
In page 11 of chapter 510, Tobi explains that by administering imagination, and the spiritual energy which forms the basis of 'yin' power... he would create shape and form from nothingness. By administering vitality, and the physical energy which forms the basis of 'yang' power... he would breathe life into that form.
As you are probably aware, a ninja's chakra is composed of a balanced mix of his spiritual and physical energy. These two components are Yin and Yang, respectively.
The Yin and Yang nature seem to be the basis for most jutsus that do use nature alteration, but are non-elemental (e.g. genjitsu). It seems that Chakra is formed when combining spiritual and physical energy. Some people are more suited with one over the other, which might determine whether their balance of Yin and Yang naturally leans towards either side. Therefore not all jutsus require an exact balance of these two Yin and Yang. Instead varied proportions of the combined forces by different means based on the intent of the user determine the jitsu.
Inton (Yin Release)
Jutsu like genjutsu and the Yamanaka Clan's psychic jutsu are spiritual and which focus on the mind. So they must use Yin and Yang nature alteration, emphasizing the Yin. These all seem to be jutsu which focus on extending your own chakra to your opponent and connecting your mind (e.g., implanting an illusion) to theirs. Whether it’s to implant an illusion, read their mind, or project thoughts etc. This basically uses spiritual chakra as a manifestation of your mind.
Youton (Yang Release)
Jutsu like Shadow Clones, medical jutsu, and the Akimichi Clan's size jutsu are physical. They focus on manipulating physical substances or creating them from chakra. They must use Yin and Yang nature alteration, emphasizing the Yang.
Yoton (Yang Release) creates life force. Inton (Yin Release) materializes imagination. Onmyoton/In'youton (Yin-Yang Release) is a combination of both Yinton and Yangton (you could say the Sage of the Six Path's unique application of this is a bloodline ability). It allows one to imagine something and materialize it without giving up anything but chakra. In other words, giving form to one's imagination.
Yin + Yang = Chakra
Yin = Spiritual Energy
Yang = Physical Energy
Yin Release = Uses spiritual energy extending the mind
Yang Release = Uses physical energy extend the body
Yin-Yang Release = Yin Release + Yang Release