A fluid, in contrast to a solid, is a material with no fixed shape that constantly deforms when acted upon by an outside force. You probably think of things like water and coffee as fluids; however, gases, such as air, and substances like motor oil are also fluids. The property that causes them to seem very different from each other is viscosity. Viscosity is the measure of a fluid's resistance to flow because of its internal friction. This is why honey is much harder to squeeze out of the bottle than ketchup. It also describes why it is more difficult to move in a swimming pool than it is on dry land.
Fluid friction is the force that resists motion either within the fluid itself or of another medium moving through the fluid. There is internal friction, which is a result of the interactions between molecules of the fluid, and there is external friction, which refers to how a fluid interacts with other matter.