PLINQ was added in the .NET 4.0 Framework as an extension to LINQ.
It allows you to add .AsParallel to your LINQ to attempt to execute the query using as many processors as possible. Neat, but you still need to know a bit about whether your algorithm is "parallelisable" - it isn't magic.
It basically removes the need to manage a thread pool and manages synchronising the results coming back from each thread - normally without the parallel extensions library you would have to do this manually.