I know the first time I ever heard the word warlock in connection with witches. It was, oddly enough, an episode of the Waltons (a show I never liked) that a baby sitter was watching. Though more likely it came from an episode of Bewitched (a show I did like).
Once I started doing research I soon discovered that "warlock" really isn't the male version of a witch. Witch is the male version of witch. "Warlock" actually comes from the word wǣrloga, which means “oath breaker”. I also read that warlock is actually an offensive term to most witches. So my first writings had warlocks as evil witches.
When I opted to use the term warlock in my 2003 Liber Mysterium I decided that Witches and Warlocks were similar classes. So I decided that at some point in their shared history, warlocks broke free from witches ideologically and were branded traitors. Witches often refer to warlocks as “betrayers”. This conflict, commonly known as “The Schism” is something neither side will speak of. However a poignant reminder remains, witches and warlocks have similar means of learning their magic (from Patrons or Pacts) and both have access to the same spells.
Despite common stereotypes, warlocks can be either male or female. Most warlocks are male, and this association is so strong that most people mistakenly call a male witch a warlock.
For 2013's Eldritch Witchery the Spellcraft and Swordplay rules allowed me to try something new. The witch was a type of cleric (something I did in 2nd ed, pre 1999) and the warlock was a type of wizard. So instead of being one thing (class) that split apart I made them two seperate things that came closer together. I liked how it turned out to be honest. For Strange Brew I am going back to my original idea of having the two classes seperate, witch and warlock, but have them have roots that are tied together. So there is more in common between the witch and warlock than the cleric and druid. Closer actually to the wizard and sorcerer relationship.
In Strange Brew the plan is now for the Warlock get the same spells the Witch does. They also have access to Hexes, though it is more limited. The Warlock also gets a magical based attack in the form of the Arcane Blast. There are also differences between the Patrons of the witches and the Patrons of the warlocks.
I am sure I will get questions about why not use the Warlock from the Tome of Magic or from 4e. Simple. I can't. They are not open in terms of the OGL. Plus I don't want too. Those warlocks are fine, but not really what I think of when I think of a warlock.
I feel that most warlocks out there in the gaming world now are influenced by the Warlock in World of Warcraft and other MMORPGs. I want to go back to much older sources and build my warlock from that.