What is the name of this activity? I've seen it being used by some Google apps, as well as some others, always at the first opening of the app. It usually presents instructions about how to use the app (in a high level) and its features.

Navigation consists of swiping left/right and, when on the last screen, an 'Ok' button appears to end the widget's presentation.

What is this activity's name, and is it implemented in any android UX library?
Not implemented as a UX library, yet.

3rd Party Solution: https://github.com/apl-devs/AppIntro

DIY: You can create your own UI and can leverage ViewPager for the effect.