Magical energy is a fundamental force of the universe which defies conventional explanation. Unlike the other fundamental forces which are known to science, magical energy operates counter to the way science describes the world. The forces of magic and its energy do not follow the laws of physics or relativity in and of themselves. They are, however, quantifiable. Scientific means can detect and quantify the effects of magical energy if not the energy itself.
Sources
Magical energy exists everywhere, there are focal points, between these focal points a network of lines which form a network of magical energy. This energy flows between points like unseen rivers of energy.
This leaves distinct sources of magical energy.
Wells
Wells are places where magical energy is in abundance and springs forth from the fabric of reality. Many ancient peoples built monuments and markers for these sites, while it was once thought that these markers were the causes of a well and that by building another similar monument one could create a well, this is not true. Wells exist, but their formation has never been witnessed and its mechanisms are unknown.
Ley Lines
Ley lines run between wells connecting to one another. Most of the time a well is only connected to its nearest neighboring wells in a 360 degree arch around the well with a distance which is limited by the power of attraction from one well to another. The ley lines server as mediators where energy balances and passes between two wells and maintain and equilibrium of energy in the region.
There is no known method to block or otherwise alter the path of a ley line. Energy may be drained or even injected into the line, but the ley line will remain intact unless some natural process causes a disturbance in the connection between wells.
Radiating
Magical energy radiates from the fabric of reality naturally, and thus it is all around us. Any given area of space has fluctuating magical energy at any given time. While a nearby ley line or well (nexus / focal point) may augment the natural magical energy present the magical energy does not come solely from such things. Ley lines and wells mediate the overall energy of a region, keeping it in balance with surrounding regions.
Effects
Magical energy can be used for a variety of purposes but only by people or things that can interact with it. While every living thing has some ability to sense and even manipulate magical energy around them it is important to note that some people, animals, or objects can do so more than others.
Animals and people who are unaware of what they are doing use magical energy out of instinct rather than purpose. The effects are usually like flashes in a pan and in line with the natural fight-or-flight response. This is not, however, the only inmate use of such energies. In people a sufficiently extreme shift in emotional and physical stress can cause the body to draw in and use magical energy, though again, this follows instinct rather than purpose.
Areas of high magical energy tend to be more fertile, plans grow better, weather seems to be more potent (for good or bad) and life seems to be attracted to such areas. Insects, animals, and people are almost always drawn to a magical well following the path of ley lines. This attraction forms roads and such along the strongest ley lines. Most cities have a well nearby - the oldest cities have a well at the very heart of the city.
Human Influence
Human emotion has a strong influence on magical energy. Enough people gathered around a weaker well can make that well strong just by their presence, which will then tend to draw more people until an equilibrium is found.
Usage
The purposeful usage of magical energy is difficult. Most people are unaware of its existence in all but the most subconscious sense. About 1/100 of the population has a stronger ability to sense the energy around them and become somewhat conscious of it, though the brain almost always interprets this awareness as a kind of emotional hot and cold sense when going from place to place.
Further around 1 out of every million people have some ability to become aware of the magical energy around them. These few people can learn (with great effort) to sense and use magical energy. It is a difficult process and must be an act of strong emotion - someone cannot learn to use magical energy just because someone tells them to - they have to have a driving force from inside which makes them want to learn. This kind of person can take a decade to really learn how to sense and feel the magic around them and longer to control it, manipulate it, use it.
Finally about once every other generation a handful of people are born for which this conscious awareness of magic is turned on from birth. For these people magic is always present and they often teach themselves to use it in some way when they are children. By adulthood many have pushed this away in an effort to conform to those around them. These people, however, can always learn and should such a person embrace what they can do - they can become very powerful turning the natural small uses of magic into an art form to do great things.