Witches are humans that posses a certain flair for magic. Like psychics, these abilities generally run in families though is not always the case as anyone with enough patience may tap into these abilities. A witch may practice Wicca, but it is not a requirement. Nor is it a guarantee that every practitioner of Wicca is a witch, since Wicca is a religion and has very little to do with tapping into the magic potential one may posses.
Most true witches keep their abilities to themselves or their coven, though many other supernaturals know of their existence, and often times hire them to provide certain spells or wards.
A witch's power is not unlimited. They can utilize so much magic before draining themselves, and thus the more they practice a certain spell within a 48 hour period, the weaker it will become. While each witch may learn any spell they desire, they must find someone who knows the spell they seek, and if it is not part of their usual sphere they may have extreme difficulties with it.
Spheres allow a witch to focus his or her energy into an expertise, making them much more valuable to a coven. Known spheres on TrueBlood: Dallas are "Elemental", "Illusion", "Protection", and "Warding".