The form of magic used by most human sorcerers is known as the magic of the Old Religion. It is the magic of the earth itself, the essence which binds all things together and maintains the balance of the world.
Rules and Beliefs
The magic of the Old Religion is considered to be the essence of the earth itself. It binds all things together and is part of the very fabric of the world, which gives it the power to made solid what is in the mind, and un-solid what is already solid (Le Morte d'Arthur, The Wicked Day, The Diamond of the Day).
Because of this, the Old Religion is not considered to be a simple belief or worship, but an omnipresent force that holds everything in balance. It is unknown whether this force is considered to be animate or inanimate. Nimueh once told Merlin that the Old Religion did not care who lived and who died as long as the balance of the world was restored, but it's unclear whether she meant that it was animate force with a will of its own that would choose whatever sacrifice it wished, or an inanimate force that could be directed to choose whatever sacrifice she wished (Le Morte d'Arthur).
Balance is a key tenet of the magic of the Old Religion. In order for something to be given, something must be taken in return. This principle is especially important to rituals relating to the power over life and death or blood sacrifices, both of which require a life to be taken (Le Morte d'Arthur, The Darkest Hour).
Though not nearly as drastic, the principle of balance can also be seen in the casting of spells. When a sorcerer uses magic, the strength and amount of magic that they used often appears to correspond to the amount of energy that they expend. For example, at the end of Merlin and Morgana's magical duel, Merlin summoned a powerful whirlwind that threw Morgana through the air. The effort of casting such a powerful spell took an immediate toll on the warlock, who immediately collapsed from the effort (A Servant of Two Masters).
Practitioners and Followers
The Old Religion is, in fact, the very essence of nature, of the universe and humans who possess the gift of harnessing this energy to their own can shape it at their will. The more they can get in contact with the power of the Old Religion, the more they can achieve and conjure into this world. Therefore creatures of the Old Religion, like Merlin, possess an incredibly strong power, for this very energy lies within them. Merlin, in the episode The Sword in the Stone: Part Two, declared that he was born with magic, possibly indicating that this power is part of him. The High Priestesses or High Priests, because they dedicate their entire existence to serve the Old Religion, have a great access to its power too. In fact, Nimueh, Morgause and Morgana all showed incredibly strong magic.
Unless the sorcerer or sorceress is extremely highly gifted (or his emotions are heightened), like Merlin or Mordred, humans must channel their power with rituals, spells and other instruments of magic. In Series 3, Morgana, having only learnt how to control her magic but not how to use it, had to rely on an object of some sort - such as when she tried to kill Arthur by using a Phoenix Eye and a poppet (The Eye of the Phoenix) - as the central focus for her spells rather than simply conjuring something on her own.
Magic, however, can be performed in other ways, when the conjurer is linked to the power of the Sidhe (Sidhe magic) or trolls (Troll magic). But the Old Religion still remains the source of every power.
A particular form of magic was practiced by the followers of the Old Religion while they were praying the old gods. When Sefa arrived at an ancient building (possibly a place of worship) in the woods outside Camelot to meet her
father Ruadan, she heard him recting a kind of spell ("Crungon walo wide, cwoman woldagas, swylt eall fornom secgrofra wera, wurdon hyra wigsteal westen stapolas, brosnade burgsteall. Betend crungon hergas to hrusan. Forpon pas hofu dreorgiad, ond paes teaforgeapa tigelum sceaded. Crungon walo wide, cwoman woldagas ond paes teaforgeapa tigelum sceaded hrostbeages hrof..."), which was probably the same call for victory over the enemies of the Old Religion that Gaius found written on a piece of parchment, hidden inside an amulet that Ruadan wore around his neck (Arthur's Bane).