What is DOM?

In React, the DOM (Document Object Model) is the underlying data structure that represents the structure of the web page.

The DOM is a tree-like structure that defines the relationship between HTML elements.

How I make/define structure in React Js?

we create or represent DOM in React by JSX and JavaScript code. After created then reacts create a virtual representation so called Virtual DOM.

What is Reconciliation?

The Virtual DOM is a lightweight copy of the actual DOM that React uses to keep track of changes to the component. When you make changes to the component, React updates the Virtual DOM to reflect those changes. It then compares the Virtual DOM to the actual DOM and only updates the parts of the DOM that have changed. This process is called Reconciliation. This helps to improve the performance and speed of React applications.