The Task Parallel Dataflow (TDF) library is built upon the existing Task Parallel Library (TPL) included in the .NET 4.0 Framework. Although the TPL provides a lot of functionality to help parallelize an application, it doesn't make it overly easy to tackle complex parallel use cases such as consumer/provider and agent-based models.
The TDF, on the other hand, provides higher-level abstractions in the form of generic code blocks that generate and schedule the needed Task objects to handle simple to complex data flows.
http://visualstudiomagazine.com/articles/2012/08/31/a-parallel-dataflow-primer.aspx