You add the forces (bearing in mind they can be in opposite directions), and any force that is left over goes into accelerating the object.
If you drop something from being stationary, initially it starts to fall with acceleration g (9.82 m/s/s). But as it goes faster, the air resistance increases in the opposite direction to gravity. Eventually the forces are in equilibrium and it does not accelerate any more, so it has constant speed. That is called its terminal velocity.
When you throw something upwards, the air resistance and gravity are added, because they are both acting on the object downwards. So the upwards velocity is reduced much more quickly than due to gravity alone.
In the absence of air resistance, the up and down trips are nicely symmetrical, and if there is any horizontal velocity you get a perfect parabola. But with air present, the frictional energy loss makes the times and speeds for the up and down parts of the journey quite different.