To be frank I heard about Roslyn a few days ago. if you watch this presentation by Andres Hejlsberg, you will also find out that It's going to be a big jump in .NET platform. Roslyn is a project that extends the capability of the compiler by giving the developers to access them. access to the compiler means that the developer can easily extend it and optimize some parts of the process by being able to directly interact with code that is being manipulated. It provides a set of API for extending the C# and visual basic compilers as a service. (F# is not included in Roslyn project)
Roslyn includes versions of C# and VB compilers written in the language themselves. It exposes modules for syntactic analysis of code, semantic analysis. Roslyn provide easy access to the information it gathers during the different stages of the compilation process. If you take a look at the Andres's presentation you will see that language services are task parallel library and multi-threading when calling the compiler API.
Roslyn provides you to create your own scripting sessions, using C# and VB.NET. It also other possibilities, you can write your own code analysis and manipulation tools around the Visual Studio to enable visual studio enhancement. Notice that because of fully rewrite the compiler by C# and VB you will be able to do all of this by C# or VB. Roslyn is not all about bringing new feature to C# and visual basic just like other series, it is much more than that. Both compiler and the language of compiler will be available. using C# and VB.NET as scripting languages, use a compiler as a service in your own applications for code related tasks, develop better language and IDE extensions, etc.