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To get started with ReactJS, you need to understand HTML, CSS, and JavaScript. Then, you can set up a React development environment using tools like Node.js and npm (Node Package Manager). You can create a new React project using create-react-app or a similar tool. Once set up, you can start building React components and defining the application's behavior.

React components are the building blocks of React applications. A component represents a part of the user interface, and it can be a small piece like a button or a larger one like a form. Components can have their state, properties (props), and lifecycle methods.

Functional components are simple JavaScript functions that receive props as input and return React elements. They are stateless and do not have their internal state. Class components, on the other hand, are ES6 classes that extend React.Component class. They have their state and can use lifecycle methods. With the introduction of React Hooks, functional components have become more powerful and can now handle state and lifecycle functionalities, making class components less commonly used.

React Hooks are functions introduced in React 16.8 that allow developers to use state and other React features in functional components. Before Hooks, state management and lifecycle methods were primarily available in class components. Some standard Hooks are useState, useEffect, useContext, and more.

The JSX (JavaScript XML) is an extension to JavaScript used in React. It permits developers to make HTML-like syntax in JavaScript code. JSX makes that easier to describe and visualize the UI components more declaratively. Babel or a similar transpiler converts JSX into regular JavaScript code browsers can understand.

React components can have their state-managed through the useState or useReducer Hooks. The state represents data that can change over time and influence a component's rendering. When the state changes, React automatically re-renders the component and its children.

Virtual DOM is a concept in React that improves the performance of updating the actual DOM. Rather than directly updating real DOM on every change, React makes a lightweight copy of the DOM called the Virtual DOM. Then React matches the earlier Virtual DOM using new and calculates minimal changes required to update a actual DOM, making the process more efficient.

You can pass data from parent to child components using props in React. Props are like function arguments that you pass to a React component when you use it in JSX. To share data between sibling components or components at different levels, you may use techniques like lifting the state or implementing a state management library like Redux.

React Router is a popular library for handling navigation and routing in React applications. It allows developers to define different routes for different components, enabling the creation of single-page applications with multiple views.

To handle forms in React, you can use controlled components. In controlled components, the form elements like input, textarea, and select are controlled by React state. The state is updated when the user interacts with the form elements, and the component is re-rendered with the new data.

To optimize React application performance, you can follow these best practices:

• Use the Virtual DOM efficiently by minimizing unnecessary re-renders.
• Avoid binding functions in render to prevent unnecessary function creations.
• Use React's built-in shouldComponentUpdate lifecycle method or React.memo for functional components to prevent re-renders when unnecessary.
• Code-splitting to load only the necessary parts of the application when needed.
• Use production builds for smaller bundle sizes.
• Implement server-side rendering (SSR) to improve initial load time and SEO.