Whether it's mounted on a plane, a ship, or anything else, a radar set needs the same basic set of components: something to generate radio waves, something to send them out into space, something to receive them, and some means of displaying information so the radar operator can quickly understand it.
The radio waves used by radar are produced by a piece of equipment called a magnetron. Radio waves are similar to light waves: they travel at the same speed—but their waves are much longer and have much lower frequencies. Light waves have wavelengths of about 500 nanometers (500 billionths of a meter, which is about 100–200 times thinner than a human hair), whereas the radio waves used by radar typically range from about a few centimeters to a meter—the length of a finger to the length of your arm—or roughly a million times longer than light waves.
Both light and radio waves are part of the electromagnetic spectrum, which means they're made up of fluctuating patterns of electrical and magnetic energy zapping through the air. The waves a magnetron produces are actually microwaves, similar to the ones generated by a microwave oven. The difference is that the magnetron in a radar has to send the waves many miles, instead of just a few inches, so it is much larger and more powerful.