A navy is a military force made up of boats and ships that are equipped for defense and fighting. If you always get seasick, you might think about joining an army instead of a navy.
During a war, a country's navy does the important job of protecting its shores, as well as attacking the enemy's coasts. A navy is made up of battleships, submarines, and smaller vessels, as well as planes that can land on aircraft carriers or on water. The color that's also called navy, a dark blue, was the original color of the British navy. The Latin root of navy is navis, "ship."