The key difference between becquerel and sievert is that becquerel is a unit used to measure the activity of a quantity of radioactive material whereas sievert is a unit used to measure the health effects of ionizing radiation.
Becquerel and sievert are units of measurement. Both these are SI derived units. SI derived units are units of measurement that are derived from the seven base units specified by the International System of Units (SI units). These seven base units are kilogram, seconds, kelvin, ampere, mole, candela and meter.