This article only refers to magic as it is referred to in the Tenchi Muyo! Ryo-Ohki continuity and has no bearing on the Magical Girl Pretty Sammy shows or any other continuity.
Magic in the Tenchi Muyo! Ryo-Ohki continuity is a power much different than other abilities in the series and is actually considered more of an ancient technology than a spiritual power.
Background
The specific origin of magic as a form of technology has not been explained fully, but it is known that it is more than ancient. Magic was developed in another universe entirely, one that no longer exists, how it was transferred to another universe is unknown. In the current galaxy, however, it was first discovered billions of years ago by a scientist researching a satellite moon of a frozen barren planet on the far edge of a solar system. Despite it being far from the sun, the moon had a thin atmosphere, and the surface of the planetoid was warm. The reason for this was a moss growing on the surface. The scientist took a sample, and from this moss, magic was spread.
Magic inhabits certain plant life and can easily be spread in this way. Like on this first moon, it can survive most anywhere and another way it is spread is by meteor landing. The moss travels through space and strikes a planet with an appropriate atmosphere to thrive. Even if there is no intelligent life on the world, magic can produce intelligent life forms within just a few thousand years. The first magic creatures in this type of environment are plant-based beings, like Dryads.
If there are already creatures on the planet, they may develop magic organs and become magical creatures like goblins, dwarves, or beasts like a chimera. Humans or other intelligent beings can also survive and thrive in a magic environment as well.
Magic plants cannot coexist with technology, the plants will overpower and destroy any tech it touches. In order to have a technological base on a magic planet, it has to be in a desert where the magic plants cannot grow. Magic didn't always react hostilely to technology, but, when it was discovered, someone tried to "clean up" some of the magic moss with technological means and ever since it perceives it as a threat. It may be possible to "reprogram" the plants not to attack anymore. Traditional technology and magic are usually very incompatible. Technology tends to stop working or gets taken over by magical plants when mixed. Sajyu, a member of the magical Dryad tribe, is a captain of a Renza Federation ship who is dependent on contact with magical plants to live. In order for this to work, the plants have to be keep isolated from the ship's tech in a floating sphere, where she resides.
It seems that magic doesn't always take over a planet though. Some planets lose their magic plants and magic becomes folklore.
There are two types of magic. "Mana" is the magic in the plants and environment. "Ena" is magic from within a person/creature. Usually, ena needs to be replenished by mana, but if a person has a high enough capacity to store ena, they can use it anywhere. Ena also has no incompatibility with technology.
Tsukiko Masaki developing a magic organ (yellow and green).
Magic beings and creatures can only use magic by developing a "magic organ" within their body. This usually happens to children early in their life. They develop a "magic cold" as the organ forms and the longer the cold lasts, the more powerful the person's magic will be. Most magic colds last less than a week, any longer signifies a great magical capacity.
Usually, a person from outside of a magical environment cannot form a magic organ (unless quite young). Also, if a person from a magic environment leaves such a place, their Ena depletes, and the magic organ becomes useless until they return to a magic environment. A notable exception is Seina Yamada, who not only developed an organ, despite being a young adult at the time, but his cold lasted for weeks, emaciating his body, but producing a truly prodigious level of magical capacity. Seina's capacity for ena magic is so great that it is naturally self-replenishing, and he doesn't have to worry about running out.
When Seina leaves the magic planet where he developed his organ, he creates a new magic environment within Kamidake's subspace, quarantined from the other areas.
Magic produces "spirit threads" (related to the concept of the Etheric Body, an energy subtle body, unlike the spiritual Astral Body) which extend from a magic user to wherever they go. Anywhere along these threads you can create a spirit thread storage space where you can utilize an unlimited amount of storage. You can also use this space to perform instantaneous transport from one place to another. The only limits to this ability are the limits of the user's imagination and that it has to be along an existing thread. You can't transport to somewhere you've never been before. In the episode Like Attracts Like and Becomes More Alike, Seina uses this ability to transport the entire Tabletop Island space vessel instantaneously to a location a short distance from attacking pirates.
It is also mentioned that magic users can travel between dimensions. This implies that Seina may, eventually, be key to retrieving Kenshi from Geminar and also allowing unrestricted access between the two dimensions.
Within the moss first found on the moon long ago was an intelligent consciousness or, maybe more appropriately called, an aware surveillance program. The first scientists to study the moss were actually part plant-based beings and were able to talk with it, however, their civilization eventually died and with them, it's ability to converse with anyone. It led a lonely existence for billions of years until it was able to make contact with Seina and his connected assistant, Nayuta. According to it, it's purpose is simple, to spread magic out into the universe.
It is also basically true, that, usually, an expert in technology cannot understand the workings of magic or vice versa, although one can interact with the other. Washu, the galaxy's preeminent philosopher and expert on all worldly technology was completely unable to understand the magical components of the Renzan mecha when she examined them.