What's the difference between slip angle and yaw angle (speaking about cars)? It seems they both are the angle between the actual direction of travel and the direction towards which it is pointing.
Yaw is typically just the heading of the aircraft on the compass.
Sideslip is the relative angle between the freestream flow and the aircraft yaw vector (to simplify it somewhat). Sideslip is the analog of angle of attack, but for yaw instead of pitch.