Is "while (true)" usually used for a permanent thread?

Yep, that's what you do.

But typically it's like:

bool keepRunning = true;

...

while(keepRunning){
}

Because sometimes you may like to have someone/something else to have the ability to stop you.

To elaborate a bit more, if a thread is sleeping, when the OS comes along to activate the thread, it will just check to see if it's still sleeping and if so, then just yield its timeslice.

If you leave out the Sleep and do something like

while (true)
{
    if (workAvailable)
        {
        doWork();       
        }
}

then even if workAvailable is false it will keep spinning until the OS stops it, taking up its entire slice doing nothing. Obviously that's a little more inefficient.

You can get even more complex as needed with mutexes, semaphores and whatnot, as mentioned above, but things get complex quickly with those, so you might want to use them to solve a particular problem.

Additionally You can use System.Threading.Timer. In this case, we don't have to use the Sleep method. Simple example:

public sealed class TimerTask
{
    private Timer _timer;
    private int _period;

    public TimerTask(int period)
    {
        _period = period;
        _timer = new Timer(new TimerCallback(Run), "Hello ....", Timeout.Infinite, period);
    }

    public void Start()
    {
        _timer.Change(0, _period);
    }

    public void Stop()
    {
        _timer.Change(Timeout.Infinite, Timeout.Infinite);
    }

    private void Run(Object param)
    {
        Console.WriteLine(param.ToString());
    }
}

Use:

public static class Program
{
    [STAThread]
    static void Main(String[] args)
    {
        TimerTask task = new TimerTask(1000);
        Console.WriteLine("Timer start.");
        task.Start();
        Console.ReadLine();
        Console.WriteLine("Timer stop.");
        task.Stop();
        Console.ReadLine();
        Console.WriteLine("Timer start.");
        task.Start();
        Console.ReadLine();
        Console.WriteLine("Timer stop.");
        task.Stop();
        Console.ReadLine();
    }
}

Console output:

Timer start.
Hello ....
Hello ....
Hello ....

Timer stop.

Timer start.
Hello ....
Hello ....

Timer stop.