QML is basically a "declarative" language for defining dynamic UI's, built on top of the Qt Framework. It's syntax is like a nicer version of CSS, with the ability to throw snippets of Javascript in when needed to define small bits of custom logic when needed, and (if it's hosted via PyQt) you can even code up any of the more "tricky" or substantial logic in Python, then use the PyQt QObject bindings to make this available to your QML code.
Although it doesn't come with a fully fleshed out widget set that you can use (though apparently, this is only true for the PyQt4/Qt4.x that I'm still using, as they've developed a set of widgets for that purpose in Qt5), it more than makes up for that in terms of what it enables you to do instead.
Basically, if you're looking for a lightweight domain-specific language for defining/prototyping little graphical scenes, complete with dynamic behaviour (i.e. procedural animations, animation effects, and transitions in response to user input), and the ability to compose/package up you creations as reusable assets, look no further. This toolkit is for you!
