Warlocks are an evil race of magical beings that are ex-human witch analogs dedicated to evil, just as Charmed Universe witches are dedicated to good. Unlike in other occult mileaus, the term "Warlock" here is not gender-specific and so refers to witches who have gone down the path of evil, rescinding their humanity after committing an irrevocable act of evil–where one of their selfish main goals is to expand their own powers by killing witches and stealing their powers.
An evil witch differs from a warlock in that they have not killed another witch to steal their powers, but rather use magic for personal gain and are willing to kill innocents instead of protecting them. Evil witches only become warlocks when they have killed good witches to steal their powers. When warlocks have children, the children will automatically be warlocks unless one of the child's parents is mortal, as evidenced by the character Brendan Rowe from the episode "When Bad Warlocks Turn Good" wherein Brendan Rowe is a half-warlock who is resolved to remain good and become a priest due to the fact that his mother was human.
It has also been stated in the episode "They're Everywhere" that warlocks do not bleed, but this has been contradicted by warlocks bleeding in other episodes. This is a definite example of a problem with Retroactive continuity (or "retcon"). It is also established, later in the third season, that the power to blink from place to place instantly is an exclusive warlock power, which in some plotlines to come will also allow differentiation between evil witches and warlocks. The power to blink is analogous to demons' abilities to shimmer, or for whitelighters/darklighters to orb between planes and dimensions.
Read more about this topic: List Of Magical Beings In Charmed