The document discusses external ballistics and the factors that affect the trajectory of a bullet. It defines external ballistics as dealing with the motion of a projectile from the muzzle of a weapon to the target. The main factors that influence a bullet's flight are gravity, which pulls it downward, and air resistance, which reduces its velocity. In a vacuum with no air resistance or gravity, a bullet would travel in a straight line at a constant velocity. However, in reality bullets follow a parabolic trajectory due to these forces. The document also discusses other factors like velocity, spin, angle of fire, yaw, drift, and temperature that can impact a bullet's trajectory.