From the .NET team's blog post:

What is .NET for Apache Spark?

.NET for Apache Spark provides high performance APIs for using Spark from C# and F#. With this .NET APIs, you can access all aspects of Apache Spark including Spark SQL, DataFrames, Streaming, MLLib etc. .NET for Apache Spark lets you reuse all the knowledge, skills, code, and libraries you already have as a .NET developer.

The C#/ F# language binding to Spark will be written on a new Spark interop layer which offers easier extensibility. This new layer of Spark interop was written keeping in mind best practices for language extension and optimizes for interop and performance. Long term this extensibility can be used for adding support for other languages in Spark.