Definitions:

• Cappuccino: a hot beverage consisting of espresso coffee and steamed milk, often served with powdered cinnamon and topped with whipped cream.
• Latte: hot espresso with steamed milk, usually topped with foamed milk.

And since they both include espresso…

• Espresso: a strong coffee prepared by forcing live steam under pressure, or boiling water, through ground dark-roast coffee beans.

Cappuccino:

• Consists of espresso and steamed milk.
• Has a thick layer of foamed milk on top.
• Is stronger than a latte.
• Has less milk than a latte.

Latte:

• Also consists of espresso and steamed milk.
• Has a lot more milk than a cappuccino. 
• Has a shorter layer of foamed milk on top.
• Is creamier than a cappuccino.

The main difference is the milk content. Lattes have more.

In fact, according to Coffee-Brewing-Methods.com, a cappuccino is an even 1/3 espresso, 1/3 steamed milk, and 1/3 foamed milk; a latte is 1/6 espresso, 4/6 steamed milk, and 1/6 foamed milk.

We can see why a latte is milkier.

Both can consist of other flavorings, and fun toppings such as cinnamon or shaved chocolate.