It’s based on “Expression”, which was a program written by a company called Creature House. Microsoft bought it a while back, and even at one time offered a free download of a previously unreleased upgrade of Expression.

I own a pre-Microsft version of Expression, and downloaded their upgrade. It’s a nifty graphics program. It’s vector based, like Illustrator. But Expression has a nifty library of painterly effects that you can apply to the vectors. And you can alter the definitions of the applied stroke styles on the fly.

You can almost think of it like CSS for vector graphics. By modifying a few stroke styles, you could change a graphic from an impressionist watercolor look to a modern hard-edged appearance. I’m not really an artist, so I never got much use out of it. But I always thought that it was an amazingly cool product.

I haven’t looked at Acrylic (and I don’t know if I’ll bother), so I don’t know what changes MS has made from the original.