Program

A program is a sequence of instructions written by humans. The code determines which steps run under which conditions. Which path is taken depends on the input, but every rule for it is fixed before the program runs.

Mental image: a machine that works through fixed, human-written instructions; the recipe behind it is the algorithm.

Once trained, an AI model behaves like a program again when it’s used: it takes an input and produces an output. Unlike a classic program, though, its behavior comes from learned parameters, not hand-written code.

An example: A home-made spam filter with three rules. If the sender is in your address book, the email goes to the inbox. If the subject contains the word “prize” or the email has more than three exclamation marks, it goes to spam. Everything else goes to the inbox. The computer understands none of these rules; it only checks conditions. To see why an email was sorted out, you find the line that fired.

Not to be confused with an algorithm: The algorithm is the procedure itself. It can be written down without a computer, a bit like a recipe. The program is the version of that procedure a computer can run. So one algorithm can sit inside many programs.

Where you’ll come across it: Almost everywhere on a computer, from spreadsheets and browsers to traffic-light control. Everyday words for programs are software or apps. In texts about AI, it usually appears as the counterpart: code written by people versus a model whose behavior comes from training.

Explained in more depth in Program, Algorithm, Model Compared.