An InputStream is a fundamental class in the Java programming language that allows for reading data from various sources, such as files, network connections, or memory. It provides a way to handle input in a consistent manner, enabling developers to read bytes of data sequentially. InputStreams are part of the java.io package and serve as the base for more specialized input classes, like FileInputStream and BufferedInputStream. They are essential for processing data in applications, allowing for efficient handling of input operations while abstracting the underlying complexities of data sources.