Viscosity is a measure of how “thick” a liquid or gas is; i.e. its internal resistance to flow. Viscosity can also be thought of as a measure of internal friction. It can be determined with a glass viscometer, which uses a capillary tube to determine flow. Or, viscosity can be determined by a rotational viscometer, which determines the force needed to turn an object in a fluid.
Rheology is a far more intricate study of the flow of matter, including liquids, soft solids, gels, pastes and even hard solids like glacial ice, which exhibits a degree of flow. Rheology is used on complex materials that cannot be described by a single value of viscosity. Rather their viscosity transforms with shifting conditions. A rheometer is similar to a rotational viscometer but has a bigger range of applications.