Drag

Drag is a type of resistive force just like friction, but it acts in a fluid (a gas or a liquid) rather than on a solid surface.

The most common example is air resistance.

Direction of action

Like friction, drag forces act in the opposite direction to the motion of an object.

When does drag occur?

Drag occurs when an object is moving through a fluid, causing a force which opposes the motion of the object.

The reason it occurs is because the object has to ‘push through’ the fluid, which is made up of lots of particles.
Moving these particles out of the way ‘uses up’ some of the energy of the object, which causes a force which opposes the motion of the object (e.g. it to slow down).

Again, like friction, drag can only oppose an existing force or motion. It can’t create a force or motion on its own - so basically, it can only slow down existing motion rather than actually make an object move.