C# - String.Format - How To Implement Custom Formatstrings

With String.Format() it is possible to format for example DateTime objects in many different ways. Every time I am looking for a desired format I need to search around on Internet. Almost always I find an example I can use. For example:

String.Format("{0:MM/dd/yyyy}", DateTime.Now); // "09/05/2012"

But I don't have any clue how it works and which classes support these 'magic' additional strings.So my questions are:How does String.Format map the additional information MM/dd/yyyy to a string result?Do all Microsoft objects support this feature?Is this documented somewhere?Is it possible to do something like this:String.Format("{0:MyCustomFormat}", new MyOwnClass())