What is req.body in node?

I am learning nodejs and express package,during my study i get various syntax such as

var id = req.params.id;
var wine = req.body;

With my req object we are acessing body, params.id , I am not getting how they works and what result we get, please tell me about these syntax ? An reference example where they use is :

 exports.addDoctor = function(req,res){
        var doctor = req.body;
        console.log(doctor);
            db.collection('doctors',function(err,collection){
            collection.insert(doctor,{safe:true},function(err,result){
                if (err) {
                    res.send({'error':'An error is occured'});
                } else {
                    console.log('Success: ' + JSON.stringify(result[0]));
                    res.send(result[0]);
                }
            });
        });
    }

In above example our console.log(doctor) will return {} only, what is its meaning ?