What is the .NET Portable Library?

If you have been browsing the .NET documentation on MSDN lately (especially the sections related to Metro and Windows Phone development), you might have noticed that many classes and methods now have a reference to something called the Portable Library. But what is this Portable Library, and why should you care?

Portable Library is Microsoft's solution to using common .NET code (think C#) to build applications that run on multiple Microsoft platforms. Thus, this is different from traditional thinking of portable; in a more general sense, portable would mean Linux, Apple, etc. platforms (Java comes into mind).

Instead, the Portable Library takes a subset of features in the full-blown PC .NET Framework, and makes sure that the API functions you use (classes, properties and methods) are available across all supported .NET platforms, including the desktop, Silverlight, Windows Phone, Metro, and so on.

If you are interested, be sure to check the original Portable Library CTP posts from last year; for instance the announcement post from January, 2011.