Unity has had a model of a free version vs. a Pro version where many of the nicer features of the engine were available only to folks who owned a license for the Pro version. But today Unity has announced the launch of the latest version of their engine, Unity 5, along with a new license, which can be summed up in an image:

The free version is now being called the Personal edition, but all Unity 5 engine features are available for free until you make over $100,000, at which point you must buy a Professional license ($75/mo subscription or $1,500 (about 1.5% of $100,000)) but never any royalties.
More details can be found here:
http://blogs.unity3d...3/03/unity-5-launch/