Difference between Thread, ThreadPool & BackgroundWorker c#

i many time call method with the help of thread like

static void Main( string[] args )
{
    Thread t = new Thread( MyFunction );
    t.Start();
}

static void MyFunction()
{
    //code goes here
}

and some time i use ThreadPool class also like

System.Threading.ThreadPool.QueueUserWorkItem(delegate {
    MyFunction();
}, null);

but i do not understand what is the difference between calling any method with the help of thread class or ThreadPool class

so i am looking a good discussion about what is the difference between Thread & ThreadPool class.also need to know when we should use Thread class to call a method and when ThreadPool class to call any method ? if possible discuss also with sample code with sample situation.

another very important question is that if i start multiple thread then my application performance will become low or bad ? if yes then tell me why...?

now also tell me what is BackgroundWorker class and how it is different from Thread & ThreadPool class. how far i heard that BackgroundWorker class also create a separate thread to run any method. so please discuss how it is different from Thread & ThreadPool class and when one should go for BackgroundWorker class.

here is small sample code of BackgroundWorker

private void button1_Click(object sender, EventArgs e)
    {
        BackgroundWorker bw = new BackgroundWorker();

        // this allows our worker to report progress during work
        bw.WorkerReportsProgress = true;

        // what to do in the background thread
        bw.DoWork += new DoWorkEventHandler(
        delegate(object o, DoWorkEventArgs args)
        {
            BackgroundWorker b = o as BackgroundWorker;

            // do some simple processing for 10 seconds
            for (int i = 1; i <= 10; i++)
            {
                // report the progress in percent
                b.ReportProgress(i * 10);
                Thread.Sleep(1000);
            }

        });

        // what to do when progress changed (update the progress bar for example)
        bw.ProgressChanged += new ProgressChangedEventHandler(
        delegate(object o, ProgressChangedEventArgs args)
        {
            label1.Text = string.Format("{0}% Completed", args.ProgressPercentage);
        });

        // what to do when worker completes its task (notify the user)
        bw.RunWorkerCompleted += new RunWorkerCompletedEventHandler(
        delegate(object o, RunWorkerCompletedEventArgs args)
        {
            label1.Text = "Finished!";
        });

        bw.RunWorkerAsync();
    }