When it was originally announced, Parallel Extensions was named PLINQ or Parallel LINQ. At that time it was a LINQ provider that could automatically parallelize queries. But since then the scope has grown significantly.
One of the most important changes is that it is no longer limited to queries. Acknowledging that some algorithms cannot be naturally expressed in LINQ, there will also be a set of imperative data parallel APIs.
For example, there is a Parallel For syntax that relies on anonymous functions. Unfortunately for VB users, it isn't nearly as clean in languages that don't support multi-line anonymous functions. Another option is the Threading.Tasks namespace. This supports an advanced task manager for scheduling operations. Unlike the thread pool currently used, tasks can be related to one another. This allows one to cancel one task and have all its child tasks automatically canceled. You can learn more about tasks and task managers on the Parallel Programming with .NET blog.
There are some widely requested features that not yet present in Parallel Extensions. Joe Duffy lists some of these including verifiable thread safety and automatic parallelism. Another feature being considered is the use of graphics processors for general-purpose uses.
The CTP for Parallel Extensions requires .NET 3.5.
