Warlocks are rarely seen on the Discworld, described by some as 'male witches' and by Granny Weatherwax as 'silly men'.
A warlock would be a man who instead of choosing the showy, geometrical magic of wizardry, chose the 'headology' and natural magic of witchcraft. Granny does not hold with this, believing men to be incapable of thinking in the right way. It's possible that if ever a warlock did show up, she'd quickly make him see the error of his ways, unless a Warlock knew the witch-like tact in knowing when not to be a witch. Granny, unfortunately, narrowly missed the arrival of Geoffrey Swivel and had no opportunity to change her mind (and little inclination, of course).
See Wikipedia for the Roundworld version of Warlocks