Input functions are programming constructs that allow users to provide data to a program. They enable interaction by capturing user input, which can be processed or stored for further use. Common input functions include input() in Python and Scanner in Java, which read data from the keyboard. These functions can handle various data types, such as strings, integers, and floats, depending on the programming language. Input functions often include error handling to manage invalid entries, ensuring that the program runs smoothly and efficiently. This makes them essential for creating user-friendly applications.