Hiya,
You will get good altitude and vertical speed data when you have a good 4 (or more) satellite fix. Great if you’re in an airborne object (plane, balloon, rocket…), probably not so useful if you’re in a land based object (car, bike, person…).
As far as I’m aware, the receiver does all the hard maths for you - so the horizontal speed is just that - speed in the horizontal plane - which is all most people want for a land based object (i.e. am I going faster than the speed limit).
Heading is the calculated true (not magnetic) compass heading - done by the receiver by using successive fixes and approximations. Note that you need both speed and heading to get velocity.
ciao, Dave