You can think of Express as a utility belt for creating web applications with Node.js. It provides functions for pretty much everything you need to do to build a web server. If you were to write the same functionality with vanilla Node.js, you would have to write significantly more code. Here are a couple examples of what Express does:

• REST routes are made simple with things like
  • app.get('/user/:id', function(req, res){ /* req.params('id') is avail */ });
• A middleware system that allows you plug in different synchronous functions that do different things with a request or response, ie. authentication or adding properties
  • app.use(function(req,res,next){ req.timestamp = new Date(); next(); });
• Functions for parsing the body of POST requests
• Cross site scripting prevention tools
• Automatic HTTP header handling
  • app.get('/', function(req,res){ res.json({object: 'something'}); });

Generally speaking, Sinatra is to Ruby as Express is to Node.js. I know it's not a PHP example, but I don't know much about PHP frameworks.

The accepted answer is really old (and now wrong). Here's the information (with source) based on the current version of Connect (3.0) / Express (4.0).

What Node.js comes with

http / https createServer which simply takes a callback(req,res) e.g.

var server = http.createServer(function (request, response) {

    // respond
    response.write('hello client!');
    response.end();

});

server.listen(3000);

What connect adds

Middleware is basically any software that sits between your application code and some low level API. Connect extends the built-in HTTP server functionality and adds a plugin framework. The plugins act as middleware and hence connect is a middleware framework

The way it does that is pretty simple (and in fact the code is really short!). As soon as you call var connect = require('connect'); var app = connect(); you get a function app that can:

1. Can handle a request and return a response. This is because you basically get this function
2. Has a member function .use (source) to manage plugins (that comes from here because of this simple line of code).

Because of 1.) you can do the following :

var app = connect();

// Register with http
http.createServer(app)
    .listen(3000);

Combine with 2.) and you get:

var connect = require('connect');

// Create a connect dispatcher
var app = connect()
      // register a middleware
      .use(function (req, res, next) { next(); });

// Register with http
http.createServer(app)
    .listen(3000);

Connect provides a utility function to register itself with http so that you don't need to make the call to http.createServer(app). Its called listen and the code simply creates a new http server, register's connect as the callback and forwards the arguments to http.listen. From source

app.listen = function(){
  var server = http.createServer(this);
  return server.listen.apply(server, arguments);
};

So, you can do:

var connect = require('connect');

// Create a connect dispatcher and register with http
var app = connect()
          .listen(3000);
console.log('server running on port 3000');

It's still your good old http.createServer with a plugin framework on top.

What ExpressJS adds

ExpressJS and connect are parallel projects. Connect is just a middleware framework, with a nice use function. Express does not depend on Connect (see package.json). However it does the everything that connect does i.e:

1. Can be registered with createServer like connect since it too is just a function that can take a req/res pair (source).
2. A use function to register middleware.
3. A utility listen function to register itself with http

In addition to what connect provides (which express duplicates), it has a bunch of more features. e.g.

1. Has view engine support.
2. Has top level verbs (get/post etc.) for its router.
3. Has application settings support.

The middleware is shared

The use function of ExpressJS and connect is compatible and therefore the middleware is shared. Both are middleware frameworks, express just has more than a simple middleware framework.

Which one should you use?

My opinion: you are informed enough ^based on above^ to make your own choice.

• Use http.createServer if you are creating something like connect / expressjs from scratch.
• Use connect if you are authoring middleware, testing protocols etc. since it is a nice abstraction on top of http.createServer
• Use ExpressJS if you are authoring websites.

Most people should just use ExpressJS.

What's wrong about the accepted answer

These might have been true as some point in time, but wrong now:

that inherits an extended version of http.Server

Wrong. It doesn't extend it and as you have seen ... uses it

Express does to Connect what Connect does to the http module

Express 4.0 doesn't even depend on connect. see the current package.json dependencies section