Node.js: req.params vs req.body

I've been cobbling together code from a few different tutorials to build a basic todo app with the MEAN stack, using node, express, angular, and mongodb. One tutorial covered creating an api for GET, POST, and DELETE actions, but neglected the POST. So, I took it as a challenge to write a function that will update an existing todo. While I got the function working, I encountered an error involving req.params that I didn't understand.

Relevant Code:

Node:

In app.js

app.put('/api/todos/:_id', ngRoutes.update);

which leads to:

exports.update = function(req, res){
    var user_id = req.cookies ?
        req.cookies.user_id : undefined;

    Todo.findByIdAndUpdate(req.params._id, 
        { $set: { 
            updated_at : Date.now(), 
            content : req.body.formText
        }}, function (err, todo) {
    if (err) 
        res.send(err)
    Todo.find({ user_id : user_id}, function(err, todos) {
        if (err) res.send(err);
        res.json(todos);
    });
    });
    };

Angular:

    $scope.update = function(id) {
        $http.put('/api/todos/' + id, this.todo)            
        .success(function(data) {
                    console.log(data);
                    $scope.todos = data;
                })
                .error(function(data) {
                    console.log('Error: ' + data);
                });
  };

Jade/HTML:

form(ng-submit="update(todo._id)")
    input.update-form(ng-show="todo.updating" type="text", name="content", ng-model="todo.formText" placeholder="{{todo.content}}")

This function works fine. It updates the todo in question, and returns the entire list to be reloaded onto the page with the updated value.

However, if in the node code, I change

content : req.body.formText

to

content : req.params.formText

I get the following error as my HTTP response:

Object { 
message: "Cast to string failed for value "undefined" at path "content"", 
name: "CastError", 
type: "string", 
path: "content" }

Even while, elsewhere in the function,

req.params._id

works fine to retrieve the todo's '_id' property and use it to find the appropriate document in the database. Furthermore, when viewing the request in Firefox's developer tools, the todo object appears in JSON format under the "Params" tab.

Why does this happen? What is the difference between using req.params vs req.body, and why does the second work and the first not?