React is SQL for HTML or more specifically the Document Object Model (DOM)
A declarative language that most developers are familiar with is SQL. SQL is declarative because you declare what data you want and the database figures out how to efficiently return you that data. React is SQL for HTML or more specifically the Document Object Model (DOM). You declare what HTML and data you want and React figures out how to efficently (with the least amount of changes to the DOM) render your data to HTML.
The architecture is component-based and allows you that allow you to create new custom, reusable, encapsulated HTML tags to use in web pages and web apps.
Why is it useful?
React makes it painless to create interactive UIs on top of web standards.