The difference between a warlock and a wizard varies, depending on who you talk to, and in both cases, the terms refer to a practitioner of magic. Some people use the terms “wizard” and “warlock” interchangeably, along with terms like sorcerer and mage, while others use these terms in very specific ways. In the Wiccan community in particular, the word “warlock” has a very specific and offensive meaning.
“Warlock” is an older word than “wizard,” coming from the Old English waerloga, which means “oath breaker,” while wizard comes from the Middle English word for “wise.” The etymologies of the words illustrate the different ways in which people sometimes view them. Wizards are generally viewed as good people with strong moral codes who also offer wise advice and assistance, and many fantasy stories feature a kindly wizard who helps the characters achieve their goals. A warlock, on the other hand, is sometimes portrayed as a much darker individual.
In communities which do not distinguish between wizards and warlocks, these terms are typically used to refer generally to male practitioners of magic. It is also common to see subtle distinctions between the two which often blur the line between wizards and warlocks. In some cases, for example, a warlock may be viewed as a more advanced wizard who works on more complex magic, and some people also consider warlocks to be more corruptible, delving into darker magic which can potentially turn them evil.
In the medieval Christian community, a warlock was specifically a male witch, and the term carried all of the connotations which went along with witchcraft. Warlocks tended to be less rare than witches, but men were occasionally prosecuted and executed for witchcraft. In contrast, the concept of a wizard as a wise and friendly adviser was common in some communities. Practitioners of alchemy were considered wizards, for example, and the idea of a guiding wizard lives on in many software programs, which have “wizards” to assist users with problems.
In some parts of the Wiccan community, the term “warlock” can refer to either a man or a woman, and it is reserved for someone who has broken the codes of the community. Someone who is exiled from a coven, for example, may be referred to as a warlock. Because many Wiccans take their oaths and vows very seriously, this word is considered to be offensive, although many are forgiving of confusion from people outside their community.
http://www.wisegeek.org/what-is-the-difference-between-a-warlock-and-a-wizard.htm