Espresso + steamed milk = cappuccino.
I could stop there, but I might as well tell you that a steak is heated meat. Both statements are true but there is more to it than that. As the cappuccino formula indicates, the beverage starts with espresso. That can be a single shot of about one ounce (30ml), or a double shot of two ounces (60 ml).
The other component of a cappuccino is milk which is steamed using the steam wand of the espresso machine. The heat of the steam and the steam’s power to agitate the milk change the structure of the liquid. The fat content of the milk wraps around the proteins and creates what is referred to as microfoam. As the name implies, this is a sort of “tiny foam.” If done properly, you cannot see any bubbles. The milk increases in volume and takes on a richer, creamier texture and slightly sweeter taste. It pours as a liquid, but with more body than when first poured from the carton.
From that point, all bets are off. What is sold as a cappuccino in the USA can range from a more traditional six ounce beverage made with two ounces of espresso and about two ounces of steamed milk with a topping of dense milk foam to a much larger beverage in a large takeaway cup. In Australia it is called a flat white which is the same six to eight ounce beverage but there is no stiff foam on top, just the espresso and the steamed milk.