1. Projectile motion is the motion of an object thrown or projected near the Earth's surface where it is only acted upon by gravity. It has both horizontal and vertical motion.
2. A projectile can be launched horizontally, thrown vertically upward, or launched at an angle. Its motion can be described using horizontal and vertical velocity and acceleration components.
3. The key principles of projectile motion are that the horizontal velocity remains constant, the vertical acceleration is -9.8 m/s^2, and the horizontal and vertical motions are independent of each other.