Who the heck is Thanos?
In short - A big badass cosmic threat. He worships death, and is a "I wanna destroy the Universe" kind of guy.
Not everybody followed Avengers comics, to know who every villain in the marvel universe is.
Corollary: it is impossible for them to pick any villain that *everyone* knows.
And the guy really does look like a purple Skrull.
You can tell the difference by the ears. Skrulls have big, pointy ears. Thanos doesn't.
In standard Marvel history - Thanos is actually an "Eternal". Five million years ago, an alien race known as the Celestials muddled in the development of life on many planets. On Earth, they muddled with proto-humanity to make us sentient, and created the Eternals (who were intended to be protectors of Earth) and the Deviants (who are kind of a side effect, and not happy about it). This same meddling left normal humans with the capacity for superpowers, leading to the supers-filled Earth of the comics.