Magic is the power to affect change by supernatural means. Magic can often be split into black and white, though depending on the situation, may also be neutral. All creatures (both human and non-human) are connected to this power and are able to access it through practice and training. However, whilst anyone can theoretically cast spells and perform other feats of magic with access to the proper texts and materials, witches generally have more knowledge and understanding of the mystical energies and supernatural forces that charged the entire universe. (Source: Fandom Wiki)
Precision and understanding of the forces does affect the performance and quality. More demanding spells require focus which only comes with practice. And affecting the energies by crafting spells from scratch, or manipulating existing ones, requires significant amounts of knowledge and practice. Not all experienced witches are able to take the craft very far in this regard.
The magic used by humans can have many different focuses and is based on multiple traditions. Witchcraft, the most common, is a practice of magic that encompasses many different types of activities including astrology, divination, spell casting, and spirit communication. It includes the practices of many cultures, nations and religions as well as many books and writings from ancient time. (Source: Fandom Wiki)
Witchcraft can be innate or it can be borrowed, for example by making a deal with a demon. Generally witches are regarded as hostiles by hunters, but there are exceptions. Witchcraft is often divided into black and white, practitioners of white magic being on the good side.
Practicing magic in itself is common day among hunters and especially the Men of Letters, for whom understanding and mastering magic is a big part of the job.
In the world of Supernatural, the occult sciences and folklore traditions are the fuel for magic. Many practices and even some sigils and spells used are based on true traditions, as is Enochian magic. The way magic is practiced in the world is bound to where and by whom it is wielded.