Millibar (symbol mb or mbar) is a meteorological unit of pressure equal to one-thousandth of a bar. A bar is a c.g.s. unit of pressure (A system of units based on the centimetre, gram, and second) equal to 1,000,000 dynes per square centimeter, or 100,000 pascals (symbol Pa). Thus one millibar is equivalent to 100 pascals or one hectopascal.
Hectopascal (symbol hPa) is a SI unit, the international system of units now recommended for all scientific purposes. SI units have now replaced c.g.s. units and Imperial units.
However, conversion is easy. 1000 hPa are equal to 1000 mbar, which is equal to 750 mm of mercury in a barometric column, which is 0.987 of the average atmospheric pressure, which on global average is 1013 millibars or hectopascals.
While hPa is used in meteorology most weatherforecasts quote atmospheric pressure in millibar. Why, confused? It might just sound a little nicer.