This is over simplifying it, but Express.js is to Node.js what Ruby on Rails or Sinatra is to Ruby.
Express 3.x is a light-weight web application framework to help organize your web application into an MVC architecture on the server side. You can use a variety of choices for your templating language (like EJS, Jade, and Dust.js).
You can then use a database like MongoDB with Mongoose (for modeling) to provide a backend for your Node.js application. Express.js basically helps you manage everything, from routes, to handling requests and views.
Redis is a key/value store — commonly used for sessions and caching in Node.js applications. You can do a lot more with it, but that’s what I’m using it for. I use MongoDB for more complex relationships, like line-item <-> order <-> user relationships. There are modules (most notably connect-redis) that will work with Express.js. You will need to install the Redis database on your server.
Here is a link to the Express 3.x guide: https://expressjs.com/en/3x/api.html