Pitch is rotation around an axis parallel to the wings, thus the nose and tail both pitch up or down.
Roll is rotation around an axis drawn through the body of the vehicle from tail to nose.
Yaw is rotation around an axis orthogonal to the pitch and roll axes. If an airplane model placed on a flat surface is spun or pivoted around the center of mass (coordinate origin) it would be described as yawing.
Aerospace engineers develop control laws and control systems to allow pilots to control their aircraft in the three dimensions described above.