A free body diagram shows one object on its own, with an arrow for every force acting on it. The length of each arrow shows the size of the force and the direction of the arrow shows the direction of the force. Forces such as weight, drive, drag and friction are all drawn this way.
When more than one force acts, they combine into a resultant force, the single force that would have the same effect as all of them together. For forces along the same line, work out the resultant by adding the forces that point the same way and subtracting the forces that point the opposite way. Forces in the same direction are added, and forces in opposite directions are subtracted.
A car has a driving force of 2000 N forwards and a drag force of 800 N backwards. The resultant force is 2000 − 800 = 1200 N forwards. A skydiver just after jumping has a weight of 800 N downwards and air resistance of 200 N upwards, so the resultant is 600 N downwards.
A resultant force that is not zero changes the motion of the object. If it acts in the direction of motion the object speeds up, and if it acts against the direction of motion the object will slow down.