Drag and lift are the forces that act on a body moving through a fluid, or that act on a stationary object in a flowing fluid. We call these aerodynamic forces if the fluid is a gas like air, or hydrodynamic forces if the fluid is a liquid.
The video below is an animated introduction to aerodynamic drag. To learn about lift check out the page Understanding Aerodynamic Lift.
There are two main causes of drag – the first is the pressure distribution around the object. If flow separation occurs as the object moves through the fluid an area of low pressure is created behind it, which can significantly increase the drag force on the object. The other component of drag is caused by the shear stresses acting on the object. This component is most significant for streamlined bodies, because the surfaces of the object tend to be more aligned with the direction of flow.