Hummingbird is the overall search algorithm, just like a car has an overall engine in it. The engine itself may be made up of various parts, such as an oil filter, a fuel pump, a radiator and so on. In the same way, Hummingbird encompasses various parts, with RankBrain being one of the newest.

Google doesn't say anything definite about Hummingbird. It’s a sophisticated part of their ranking engine. That’s all we know.

Did you even read the article?

They have control points +/- that tweaks the algo on the fly matching relevant search queries to relevant keywords/definitions.

Basic contextual search.

In simple words, you can say that RankBrain is a system developed by Google, where it is designed to understand the intention of a user and what the user is actually trying to find, and gives the search results based on that information.

RankBrain seems to deal with parts of the search that don't involve ranking. Well, not directly at least. It's an AI algorithm that they use to parse complex queries.

We've all been experiencing the results from this algo for a while. I've recently posted several rants about Google not being about exact keywords. This change just means that the contextual and semantic parsing is even more pronounced.

RankBrain is one of the “hundreds” of signals that go into an algorithm that determines what results appear on a Google search page and where they are ranked.
According to Google it’s the third most important factor in determining the results that appear in your search queries.
At the moment, Google’s Rank Brain is reported to be responsible for 15% of all online searches and is now one of the most important signal out of the hundreds of signals used to determine search ranking.