A core idea of Taoism is the concept of Sancai, or the trinity of the universe. The number three reflects the generating power, the imbalance of forces needed to move forward.
In the Tao Te Ching we see:
The Tao gives birth to One.
One gives birth to Two.
Two gives birth to Three.
Three gives birth to all things.
So from the Tao (the underlying All) comes the Wuji (Oneness) from which comes the Taiji (the balance of Yin and Yang) which produces the Three, which becomes the Ten Thousand Things (the entire Universe in all its multiplicity)
We see this trinity in daily life, in the way we separate upper, middle and lower, or the past, present and future. In Qigong techniques, you find Posture, Breath and Visualization as the three main tools of Qi development.
Within the body we find the Lower, Middle and Upper Dantian, with the Upper and Lower Dantian (in the head and belly) issuing energies which are transformed within the third Dantian (near the Heart).
The body itself can be seen as a unified trinity, that of the Jing Body, the Qi Body, and the Shen Body. Some traditions define upwards of nine types of bodies, ending with the greatest and more diffuse of our bodies being the Universe itself. But these bodies are not accessible to most people. They require skills that only very experienced meditators have in order to simply be aware of them, let alone have any influence over them.
So the Three Bodies remain the main way to describe ourselves.
The Jing Body is often described as the essence of the body. I find that less than helpful, since the next questions is then: what do you mean by essence? A more useful description would be the Chemical Body, or even the Wet Body. The Jing Body is the sum of the various gooey things within that transmit chemical energy (blood, bile, hormones, …) and kinetic energy (muscles, tendons).
The Qi Body is what it sounds like: the network of Qi channels and meridians that feed the body. Qi is not accepted within Western medical science, but it can be thought of as the sum of the electric impulses that course through our nervous system. It has sometimes being called the Electric Body. I find that definition rather narrow since Qi is more closely related to our vital energy than just nerve impulses, but even if you do not believe or understand the nature of Qi, the idea of the Electrical Body can be useful.
The third body, or Shen Body, is the most diffuse. It is our spirit body, the aspect of ourselves which may survive death (if we have nourished it enough through meditation). There resides our emotions, our “soul”. These are hard concepts to define since every culture has a different definition of what a soul is.
As you can see there is a progression, from the material to the immaterial, from the solid to the ethereal. In Taoism, the Qi Body is the bridge between the Shen Body and the Jing Body, the connector between the physical body and the spirit body. There is no disconnect between body and spirit in Eastern philosophy. In the West, the body has often been described as a cage for the soul, something vile, negative, which is dragging down the soul. Within Taoism, the spirit needs the body. It would not exist without it. One is not superior to the other, they are complimentary.
The very history of Eastern martial arts illustrates this. The internal martial arts (which include Taijiquan and Shaolin Gungfu) were developed from exercises used by meditating monks who found their bodies to be wasting away from too much inactivity. The monks needed their bodies in order to continue their spiritual practice.
In Taoism, the Qi is transformed into Shen within the body. With no body, the Shen is ethereal, fragile, and disperses at the moment of death. So the Three Bodies are interrelated, interdependent of each other. Imbalance in one affects the others. The goal is integration and balance of all three.
As we do the Form or the Basic Exercises, we can bring our focus to these different bodies. I find it helpful to ask myself what body is being worked on most by each exercise. Many Basic exercises work on both the Jing and Qi bodies at once. But it is possible to emphasize one or the other body by making minute changes in the exercises.
I find that the Jing Body needs to be worked on first in order to access the Qi Body effectively. So the warm ups have a natural progression, from Jing to Qi. Similarly, the Forms (13 Postures, etc..) can be done with a focus on the Jing (applications, foot position, etc..) or the Qi (breath, Qi flow). Yes, in the best of al possible world all would be integrated, but we are only human, and many details are missed if you try to do too many things at once.
Develop the Jing Body, work on the Qi Body, then integrate. Then you will be ready to join the Jing, Qi and Shen.