Key Difference: Atmospheric pressure is a force in an area pushed against a surface by the weight of air in Earth's atmosphere. A barometer measures Barometric Pressure, which in most common instances is synonymous with Atmospheric Pressure.

One of the basic laws of physics is that everything must have mass, no matter how insignificant. As it has mass, then gravity must act upon it, and hence it must also have weight. Weight is nothing more that the force of gravity acting upon mass. This also applies to things that we generally do no consider being heavy, such as air.

Air is made up of many different types of molecules, such as oxygen, carbon dioxide, nitrogen, etc. We might not notice the weight of air on an everyday basis, but it is there, we have just gotten use to it. It is because of the weight of air that we feel wind, which is the extreme movement of the molecules in the air. We also notice the change in the weight of air as we climb up a mountain. The air gets thinner there as the molecules in the air are more spread out.

Pressure is basically the ratio of force to the area over which that force is distributed. This basically means how much something is pushing on something else. For example, say you have a heavy book on top of your head. You feel the weight on top of your head because the book is exerting pressure on you. Similarly, the air also exerts pressure against everything it touches. This pressure is called the Atmospheric Pressure.

The Atmospheric Pressure can vary depending on how much air is located above the location where the measurement is taken. Similar to how the air gets thinner as we climb higher, the Atmospheric Pressure is also reduced.

A barometer is device which allows one to measure Atmospheric Pressure. This is an important device, as changes in atmospheric pressure can forecast short term changes in the weather. A barometer in an important tool for a meteorologist, as it can help predict a storm, cyclone, typhoon, hurricane, etc.

There are many different types of barometers; the most popular one is the mercury barometer, which consists of a vertical mercury column and a wide reservoir at base. As the Atmospheric Pressure increases, it puts more pressure on the mercury in the base, which pushed the mercury up into the vertical column. Similarly, if the Atmospheric Pressure decreases, then the mercury in the column drops.

A barometer measures Barometric Pressure, which in most common instances is synonymous with Atmospheric Pressure. Barometric Pressure is measured in millibars (mb) or in inches or millimeters of mercury (Hg). Normal pressure at sea level is 1013.3 millibars, or 29.92 inches of mercury.

Pressure in general is measured in Pascal (Pa), which is the SI unit of pressure. However, Atmospheric Pressure is usually measured in atm. Normal atmospheric pressure is called 1 atmosphere. 1 atm = 14.6956 psi = 760 torr.  Atmospheric Pressure can also be measured in millibars (mb or mbar.).

Image Courtesy: kids.earth.nasa.gov, physicalgeography.net