Are they coffee?
Yes. Both a cappuccino and a latte are coffee drinks — specifically espresso drinks. Each is made by combining a shot of espresso with steamed milk (and, for the cappuccino, a thick layer of foam). They contain real coffee and caffeine.
Are they sweet?
Neither is sweet by default — there is no added sugar in a plain cappuccino or latte. However, the latte tastes slightly sweeter because it contains more steamed milk, and milk's natural lactose adds a mild sweetness. Flavored syrups are what make café versions genuinely sweet.
What is the difference, in one line?
A cappuccino is espresso with equal parts steamed milk and thick foam (strong, airy). A latte is espresso with a lot of steamed milk and a thin foam layer (mild, creamy). Same espresso, different amount of milk.