In a typical React.js application created using `create-react-app` or set up manually, you'll find the following folder structure:

my-react-app/
  ├─ node_modules/        // Dependencies installed via npm
  ├─ public/             // Static files (e.g., index.html)
  │   ├─ index.html       // Main HTML file
  │   └─ ...
  ├─ src/                // Source code
  │   ├─ components/      // React components
  │   │   ├─ Component1.js
  │   │   ├─ Component2.js
  │   │   └─ ...
  │   ├─ App.js           // Main application component
  │   ├─ index.js         // Entry point for React app
  │   ├─ ...
  │   └─ ...
  ├─ .gitignore           // Git configuration to exclude files
  ├─ package.json         // Project configuration and dependencies
  ├─ README.md            // Project documentation
  ├─ ...
 

Here's a breakdown of the key folders and files:

1. node_modules : This directory contains all the dependencies (libraries and packages) installed via npm. It's usually very large and is not typically included in version control.

2. public :
   - `index.html`: The main HTML file that serves as the entry point for your application. It contains a `<div>` element with an `id` of `"root"` where your React app will be rendered.

3. src :
   - components : This folder is where you'll store your React components. Each component is typically defined in its own file.
   - App.js : This is the main component that serves as the root of your application. It often contains the structure of your app and may import and render other components.
   - index.js : This file is the entry point for your React application. It imports `App.js` and renders it to the DOM.
   - Other files and folders as needed for your specific application logic and organization.

4. .gitignore : This file lists files and folders that should be ignored by version control systems like Git. It usually includes the `node_modules` directory, build artifacts, and other files that don't need to be tracked.

5. package.json : This file contains metadata about the project and a list of dependencies required to run the application. It also includes scripts for tasks like starting the development server, building the app, and running tests.

6. README.md : This is the documentation file for your project. It typically includes information about the project, how to install and run it, and any other relevant details.

7. Other Files and Folders :  Depending on your project's needs, you may have additional files and folders for things like stylesheets, images, configuration files, and more.

Remember, while this is a common structure, you can customize it to fit the specific requirements of your project. Additionally, if you're using a different setup (not `create-react-app`), the structure might vary slightly.