What is the basic difference between ECMAScript and Javascript?

I wanted to do a develop an application in React. And also a Android Application of the same application using ReactNative.

So should I care learning for ECMAScript?

And if not, Kindly link a book which I can study for Javascript(short introduction+ intermediate level) which will help me building applications.

Please See: this is not my first programming language, I can do programming in C++ and Java(Most of the time).