Pitch, yaw and roll are terms originating in aviation. They refer to the three different types of rotation a plane can perform when moving. These terms were adopted into 3D design and game development to more precisely define a virtual 3D environment and how things are moved in that virtual space. Pitch is the up-and-down movement of an object relative to its original position. Yaw is the horizontal left or right movement relative to its original position. Roll is a tilt to the left or right relative to its original position.
These terms are also used with camera devices to describe movements of the camera.
Also see axis.