An Input Stream is a sequence of data that is read from a source, such as a file or network connection. It allows programs to receive data in a continuous flow, making it easier to process large amounts of information without needing to load everything into memory at once. Common types of input streams include those for reading text files, binary files, and data from the internet. In programming, input streams are often used in conjunction with Output Streams, which handle data being sent out. They are essential for tasks like reading user input from a keyboard or retrieving data from a database. Popular programming languages, such as Java and Python, provide built-in libraries to work with input streams efficiently.