Event Handler

Event Handler in React JS is a JavaScript function or method that gets executed when a specific event such as clicking a button, loading a page, or submitting a form occurs. These functions are usually passed as props in a component. It allows React to handle and control user interactions like mouse click events, key press events, or form submission events through these handlers.

Here is a simple code snippet demonstrating an event handler in React:

```jsx
import React from 'react';

class MyButton extends React.Component {
handleClick() {
alert('Button has been clicked!');
}

render() {
return (
<button onClick={this.handleClick}>
Click Me!
</button>
);
}
}
```

In this example, `handleClick` is the event handler. When the button is clicked, it triggers the `handleClick` function which in turn displays an alert box saying 'Button has been clicked!'.

The event handler handleClick in this case is passed into the onClick prop using JSX syntax (wrapped in curly braces {}). React automatically provides synthetic events system that ensures events have consistent properties across different browsers. Event Handlers in React must be written in camelCase rather than lower-case of HTML.