In the magical world, witches are the fundamentalists. The experts. The scholars. They are the baseline that all other types of magic are compared against, and they were also the first of the magic-users to be discovered and documented on Earth.
A witch is a human being who has the ability to manipulate magic at will. Usually, this ability is the result of a single bloodline, training witches for generations in the secrecy of other worlds. Other times, however, witches awaken to their powers spontaneously— as was the case with the Eclipse, where magic was forcefully introduced into the fabric of reality. These magically gifted humans can master all manner of spells and techniques, making them extremely versatile. However, they are ultimately still the same person they were before. They are bound to human bodies, and human lifespans. They are also fickle, as all humans are— a trait that tends to make them bite off more than they can chew.
Witches are often categorized into subtypes based on their magical strength and style of technique. It is common to specialize between "external," "internal," and "spiritual" forms of magic, though the exact details vary wildly from place to place.
Witches are fundamentally similar to heirs, being one of the primary types of magic-users. The differences lie in the ways that their magic is manifested.
Witches are jacks-of-all-trades, and focus on the fundamentals at heart. It is common for a single witch to know hundreds of basic techniques, and just as likely for them to specialize in one or two advanced spells. Compared to heirs, they are more stable, and have a much wider scale of abilities to choose from. They are also extremely limited by time. It can take years to master a technique, and many more without a proper teacher to practice with. Heirs are all but guaranteed these teachers, and they have as many years as they could possibly need to adapt to their own strengths. It's not all bad, though. After all, you get to choose to live your life as a witch. And there are many more paths to success than you might realize.
But of course, heirs can never become witches.