Alright guys, I'll try to set you straight...
a bar is approx. equal to the atmospheric pressure on the surface of the earth. It is one pascal (Pa) , named after the famous physicist
Technically, 1 pascal is 100,000 N/m2 (newton per meter squared). That is in SI units (Systeme Internationale).
Realize, that when we measure pressure (and this is a critical point), that our pressure guage doesn't show the true pressure. It shows what we call "guage pressure." The pressure guage is calibrated to subtract off the atmospheric pressure, so that a flat tire shows "zero" pressure. But really, we have substantial atmospheric pressure all around us.
One can think of the atmospheric pressure merely as the weight of all of the gasses in the atmosphere, contained in a cylnder directly above you, pushing down on you. That is how I like to think of it. It is really what is going on. It is very cool conceptually, and most people don't know it, and even fewer "get it." It is simply gravity at work.
Cheers
-Nimbus, physics geek
p.s. For more reference, to think about what 100,000 N/m2 (meter squared) is. Think about a 1 kg block of cement. The weight is mg = 9.8 kg per meter squared. Spread that over one square meter, and you're at a pressure (force per area) of 9.8 N/m2. So 100,000 is pretty huge. The gasses in our atmosphere weigh surprisingly much. And it is all pushing on us. Very cool concepts. But it'd take (approx) 10,000 kg of gasses in our 1 square meter cylinder pushing down due to gravity to create that pressure. That is what is physically going on ....(1 kg = 2.2 pounds for you gringos)