Essentially, that's the way the program appears to work to the user, but how it works is each function has a state variable and a switch statement, so the first time it's called it might do the code for state 0, then set the variable to state 1, next time it's called it'll do the code for state 1, then it might set the state to 50 to do that code next time, or stay in state 1 while waiting for something. All the functions are called in a loop in the main function and each does a little part of what they do each time they are called so it appears as if they are running at the same time.

It's a simple concept, though hard to explain on a msg board...
--WetWired