According to Wikipedia, "A cappuccino is an espresso-based coffee drink that originated in Italy, and is traditionally prepared with steamed milk foam (microfoam)."
Traditionally, it's considered to be a "breakfast" drink, meaning that ordering a cappuccino after lunchtime, or during a meal other than breakfast, is generally frowned upon in Italy and considered one of the main giveaway signs to expose someone as a non-Italian.
Acceptable variations on a cappuccino include some cocoa or cinnamon powder on top, or using non-dairy milks such as soy milk.
There are some coffee-and-milk based drinks that look like a cappuccino, but they have different names, the most notable being:
Marocchino, smaller in size and with more cocoa:
Macchiato, espresso-sized and with just some drops of warm milk. It can be also additionally varied as Macchiato freddo with cold milk, which is typically chosen by hasty people who don't have time to let the hot coffee cool down.