Redux is a popular state management library used with JavaScript applications, particularly those built with React. It was created by Dan Abramov and Andrew Clark in 2015 and has since become an industry standard for managing complex state in large-scale applications.

The concept of Redux is simple: it manages the state of your application in a single, immutable store. Rather than passing data between components or using multiple state variables, Redux provides a centralized way to manage the state of your application. 

In Redux, state is represented as an object, which is managed by the store. The store is responsible for updating the state based on actions that are dispatched to it. Actions are plain JavaScript objects that contain information about what happened in your application. They typically include a type property that describes the action and a payload property that contains any data associated with the action.

Reducers are functions that take the current state and an action as arguments, and return a new state based on the action. They are responsible for handling the logic of updating the state based on the action that was dispatched. Reducers must be pure functions, which means that they do not modify the original state, but instead return a new state object.

Selectors are functions that are used to retrieve data from the store. They can be used to retrieve specific pieces of data from the state, or to combine multiple pieces of data into a single object. Selectors are typically used in conjunction with the mapStateToProps function in React components.

Middleware is a way to extend the behavior of Redux. It provides a way to intercept actions before they reach the reducer, or to modify the behavior of the reducer itself. Middleware can be used for a variety of purposes, such as logging, handling asynchronous actions, or implementing authentication.

One of the main benefits of Redux is that it makes it easier to reason about your application's state. By centralizing the state in a single store, you can easily see how the state changes over time in response to different actions. This makes it easier to debug your application and understand how it works.

Another benefit of Redux is that it makes it easier to share state between different parts of your application. Because the state is centralized in a single store, you can access it from anywhere in your application without having to pass data between components. This can make your code more modular and easier to maintain.

However, Redux does come with some overhead. Because you need to define actions, reducers, and selectors, there is some boilerplate code that needs to be written. This can make it more time-consuming to set up Redux in your application compared to other state management libraries.

In conclusion, Redux is a powerful state management library that can help you manage the state of your application in a more centralized and organized way. By providing a single store, actions, reducers, and selectors, Redux makes it easier to reason about your application's state and share state between different parts of your application. While it does come with some overhead in terms of setup and boilerplate code, the benefits of Redux make it a popular choice for managing complex state in large-scale applications.