Measurement involves comparing an unknown physical quantity to a known standard of the same type. Physical quantities that can be measured include length, mass, time, temperature, and electric current. The International System of Units (SI) provides standard units that are used universally, with length measured in meters, mass in kilograms, and time in seconds. Measurement systems include the centimeter-gram-second (CGS) system, foot-pound-second (FPS) system, and meter-kilogram-second (MKS) system. Fundamental units are independent units like length, mass and time, while derived units are obtained from fundamental units, such as area from length and volume from length and width.