`StringWriter` is a class in the System.IO namespace of the C# programming language. It is used to create a string buffer that can be written to, allowing developers to build strings dynamically. This is particularly useful when you need to concatenate multiple pieces of text efficiently without creating multiple string instances. The `StringWriter` class works in conjunction with other classes like `StringReader` and implements the IDisposable interface. It provides methods such as `Write` and `WriteLine`, enabling easy manipulation of string data. Once the writing is complete, the resulting string can be retrieved using the `ToString` method.