[node.js] What are "res" and "req" parameters in Express functions?

In the following Express function:

app.get('/user/:id', function(req, res){
    res.send('user' + req.params.id);
});

What are req and res? What do they stand for, what do they mean, and what do they do?

Thanks!

I noticed one error in Dave Ward's answer (perhaps a recent change?): The query string paramaters are in request.query, not request.params. (See https://stackoverflow.com/a/6913287/166530 )

request.params by default is filled with the value of any "component matches" in routes, i.e.

app.get('/user/:id', function(request, response){
  response.send('user ' + request.params.id);
});

and, if you have configured express to use its bodyparser (app.use(express.bodyParser());) also with POST'ed formdata. (See How to retrieve POST query parameters? )

Request and response.

To understand the req, try out console.log(req);.