Is it just for animations
Yes.
Or does it also present some logic like Fragments and Activities?
That depends upon your definition of "logic".
What is the difference between using it and just animating each view you'd like to animate?
Simplicity.
Are there any official samples for this?
There is one on your hard drive already, assuming you have downloaded the SDK samples.
See also Mark Allison's blog post series.
Is there any compatibility library, to use it on pre-4.4 versions?
No, but there is a community backport.