StreamWriter is a class in the System.IO namespace of the C# programming language, used for writing characters to a stream in a specific encoding. It simplifies the process of creating and managing text files by providing methods to write strings, characters, and lines efficiently. When using StreamWriter, developers can easily open a file, write data to it, and close the file when done. This class also supports automatic flushing of data, ensuring that all written information is saved properly. Overall, StreamWriter is a valuable tool for file handling in C# applications.