Selectors are custom functions that extract specific values or pieces of information from a store's state value. As the state value grows and size and complexity, the selector functions can be useful in extracting relevant information.

A simple example is as follows:

```js
// Consider that state has a dictionary of posts identified by ids, and each post object has a 'date' key
// We create a selector called 'selectPostDate', which extracts the post date based on 'id'

let selectPostDate = (state, id) => state.posts[id].date;

const postDate = selectPostDate(store.getState());
console.log(postDate);
// 6 Nov 2023
```

In this section we will learn what Redux is, why it is useful, and most importantly when it is useful. We will also learn how to set up a basic react-redux application.

In this section we will go through the fundamental concepts of Redux that are necessary to understand the workings of a Redux application. We will use code examples to understand the concepts and in the following section we will use them practically.

In the previous section we looked at the basic concepts. In this section we use those basic concepts to understand some more complicated concepts and the structure of a Redux application. In the end we will also look at the Counter application to put together all the pieces of our knowledge together.

This chapter is focused on practice hence we will utilize the knowledge we acquired in previous sections to create a practical application using Redux along with React.