Even though its not official, what apparently happened is that back in the days, Blizzard wanted to make a Warhammer-based strategy game and word goes they actually got the rights. Unfortunately, Games Workshop decided to pull the plug of this project for undisclosed reasons. However blizzard did not quit so easily. since they were kind of a small company back in the days, they decided trashing the project in which they had worked so hard would be a complete waste of resources, so instead they changed most copyrighted names and brands on the material they already had, created some different assets, and thus Warcraft was born.
Starcraft is a completely different matter because it was created years later. Most of SC races are inspired in one or more W40k races, while still keeping certain key differences along with a completely different story background. The ironic thing is, SC was originally intended to be a Star Wars game, but since acquiring the Lucasarts rights would be severely expensive, Blizzard once more decided to create something more of their own. While SC is still massively inspired by w40k, it can still mantain a certain originality of its own, so no, SC isnt a ripoff of W40k.
