In Buffy the Vampire Slayer and Angel, a witch is someone who has a great knowledge and power over mystical forces, commonly known as "magic" and informally known as "mojo" or "the whammy" to defy the laws of nature and perform fantastic magical features. The term "witch" is used to refer to a female practitioner of witchcraft, while her female counterpart is called a warlock. In the Buffyverse, magic is also called "magicks".
In the Buffyverse, becoming a witch requires a great deal of work. Even fairly accomplished witches (such as Willow Rosenberg) can take years to perfect their powers.
Spells can be performed by anyone, although witches usually have a greater knowledge and power over magic. Because anyone can use magic -- from demon to human -- can become a witch. Witches who have inherited their powers from a parent are called natural witches (Tara Maclay and Amy Madison, for example). Witches who obtain their power purely from training have no formal name and are apparently subject to magical burnouts if they overuse.
Magic is neutral and can be used for good or evil. Willow and Tara used thier magical ability to support Buffy in the fight against demons, vampires and other evil beings. Even good magic is highly corruptive and can cause addiction, an issue which was focused on in the sixth season. After the death of Tara, Willow becomes consumed with dark magic in a spur of vengeance.
A technopagan is a type of magical practitioner who incorporates technology into their witchcraft. More specifically, technopagans see the supernatural in technology and can use devices like laptops and computers to cast spells. The foremost technopagan in the Buffyverse is Jenny Calendar, Sunnydale High's computer teacher. Although she does identify as technopagan, she does not call herself a witch, noting that she doesn't "have that kind of power". Although Willow was extensively involved with computer science, she was not a technopagan.