An OutputStream is a fundamental class in Java used for writing data to a destination, such as a file or network socket. It provides methods to send bytes of data, making it essential for tasks like file handling and data transmission. The class is abstract, meaning it cannot be instantiated directly, but it has several subclasses that implement specific output functionalities. Common subclasses of OutputStream include FileOutputStream, which writes data to a file, and ByteArrayOutputStream, which stores data in a byte array. By using these subclasses, developers can efficiently manage data output in various applications, ensuring smooth data flow and storage.