Every time a witch casts a spell, he or she is manipulating energy. They are pulling energy from all around them and compressing it so tight that eventually it explodes.
Channeling is the power to draw cosmic forms of energy from other dimensions of the Universe. According to Andrew Fleming, witches are able to perform miracles by siphoning energy from the natural environment and manipulating these energies to their own ends. It is heavily implied that all humans have this power, but it lies dormant within them and can only be activated through mental concentration and meditational practice. The only exception to this are with the "Natural witches", who, according to Andrew Fleming, are born with this ability already active and available for them to use.
This power was most famously used by Bonnie, Nancy and Rochelle - who channeled the psionic powers of Sarah Bailey.