Measuring air pressure?

Question

What's the difference between measuring th air pressure in inches and in mb. My barometer has both.

Answer 1

A "bar" is the equivalent of one newton of force per square meter.  That is, the air is exerting that much force (pressure) on a given area.  A "newton" is a standard measurement of energy.

"Inches" refers to inches of mercury, which is an older way to measure air pressure.  It refers to the amount of force exerted by "a column of mercury of 1 inch in height at 32 °F".

Answer 2

No difference.  Just a different scale.  Just like saying 3 feet or 1 yard, both exactly the same length just a different measuring scale.