State is an object that determines how the component renders and behaves. It is the place where the data comes from. It allows you to create components that are dynamic and interactive.
State can be defined as key-value pair defined in each component.
It can be -- Initialized with this.state -- Updated on event calls.
When a component's state data changes, the render function will be called again to re-render the change in state using this.setState.
ReactJS - Basic 
ReactJs - Element 
ReactJS - Basic