What is Singleton design pattern ? How to implement it ?

public sealed class Singleton
{
static Singleton instance=null;
static readonly object padlock = new object();

Singleton()
{
}
public static Singleton Instance
{
    get
    {
        lock (padlock)
        {
            if (instance==null)
            {
                instance = new Singleton();
            }
            return instance;
        }
    }
}

}