A Function is similar to a subroutine in that it is part of a program which can be performed a number of times. The difference is that a function has input parameters & output parameters. For example I have developed a function which you pass in the date of your birthday - the function then calculates how many days there are until your next birthday & passes the result of this back to the program - I use this function in a Web Page which displays the number of days until the next birthdays of all the members of my family (except uncle Bert who likes to keep his birthday a secret).