Yaw is a rotation of a vessel about it's vertical axis. In practical terms, yaw is the change in course of a vessel; as such, it is generally measured in degrees. Yaw is a principal ship motion.
Tracking, or coursekeeping, is a description of a vessels tendency to maintain directional stability. To decrease a vessels susceptibility to yaw in a seaway (and thus improve tracking), larger L/B's ratios, skegs, deep keels and/or keelsons can be fitted. Similarly, to increase manoverability at the expense of tracking, increased rocker can be used. Yaw can be actively induced through the use of rudders, azimuthing propulsion, Voith Schneider drives or thrusters.