This is an Arduino Library for simple text based serial communication. It uses the convention of command <arg1> <arg2> <...> # where command is a byte value. It is meant to be an alternative to the Messenger library and Firmata.
Simple Download the library, extract the folder, rename the folder to "Arduino_NET" and place this folder in your Arduino/libraries folder. Open a new project and try out the example "simple".
There is an associated .NET library (hence the name) to provide easy API access between the Arduino and your application. Download the source code and take a look at the Example App to see how it works.