Steamworks is a set of API's made available by Valve to allow developers to create integrations with Steam.
This ranges from the simple things like getting the local player's name, to more complicated things like server lists and UGC (Workshop Stuff).
Valve's Steamwork library is written in C++. Due to the popularity of Unity a lot of game development is now done in C#.
Facepunch.Steamworks is a C# wrapper on top of Valve's steamwork library.
Steamworks.NET aims to be a faithful wrapper with the C++ version. Facepunch.Steamworks is a re-interpretation of the Api in a C# world. Here's an example
Getting Friends List on Steamworks.Net:
Getting Friends List on Facepunch.Steamworks:
You can download a release on the Github Page.