The news came direct from Notch himself, via - where else? - his Twitter:
I got an email. We've reached 100 million registered users on the original Minecraft. 14.3% conversion rate to paid accounts. Wow. :D <3
The "original Minecraft" he's referring to is, of course, the PC version, which was first publicly released back in May 2009. In the nearly five years since, the game has progressed through alpha testing, to a beta stage and now stands alone as a fully-fledged, playable game.
It's still the subject of frequent, comprehensive updates and tweaks, with major changes rolled out on a regular basis.
While it originally started out on PC, Minecraft has spread to other platforms. A console version was released for Xbox 360 and PlayStation 3 (Xbox One and PS4 are in development), while handheld adaptations are playable on Android and iOS, with PlayStation Vita support on the way. There's also a cut-down version for mini-computers like the Raspberry Pi, giving you plenty of options if you want to get your world-building on.
Jimmy the Geek